feat: discussion artifacts as wiki pages, in two new layers

A discussion leaves behind a directory of markdown somewhere outside this
repo, and the only durable home for it is the Gitea wiki. Getting it
there by hand means re-deriving the same three things every time: what
each file should be called, where it goes, and whether the page already
exists. Two skills wrap that, along the split the repo already uses.

`skills/page` is domain, offline, stdlib-only, and knows nothing about
Gitea. It imports a directory into a space under `tmp/wiki/`, titles
every file, records the result in `.pages.json`, and writes the index.
`skills/wiki` is the bridge — `wikimap.py` translates, and the transport
is `_gitea.py`, the same one the issue side uses. There is no second
transport, and `tea` has no wiki subcommand to offer one.

The Gitea wiki is flat, and that fact shapes everything

There are no directories. A title of `a/b` is stored as one file named
`a%2Fb.md`, and Gitea escapes it by rules of its own: space becomes `-`,
`/` becomes `%2F`, and a literal `-` forces a trailing `.-` marker so the
two stay distinct. `Chain decisions — DC` under two levels of prefix
comes back as `Simple-Chains%2FParked%2FChain-decisions-%E2%80%94-DC`.

So `sub_url` is the identity, it is read back from whatever the API
returned, and it is never constructed. One built by hand that is almost
right does not fail — it creates a second page and abandons the first.

And a real subdirectory committed into a wiki's git repository is a ghost:
the file exists, the API and the web UI do not see it. `folder/page.md`
in this repo's own wiki is one. Nothing here clones a wiki repo.

A title is a decision, not a derivation

Titles come from the first heading, because there is no mechanical route
from `03-q-01-do-we-know-the-chain-participant-by-name.md` to
`Q-01. Do We Know the Chain Participant by Name`. But they are derived
exactly once. A re-import replaces bodies and keeps titles, so editing a
heading cannot rename a published page — which would not rename it, it
would publish a second one.

`--retitle` opts in. It finds the prior entry by `source` rather than by
path, because the path is derived from the title and a retitle moves it;
looked up by path the page would read as new and the next push would
duplicate it. The old file goes, `sub_url` comes along, and `pushed` is
cleared — a rename can leave the body byte-identical, and push decides by
body hash alone, so a stale hash would skip the rename forever.

Ordering is a `NN-` file-name prefix and never reaches the title. `00-`
means "this is the directory's own page", and that page is named for the
directory, not for its own heading: a child's title has to extend its
parent's exactly, and `ideas/00-intro.md` opens with "Ideas for chain
business requirements".

Path collisions are reported and never resolved. Picking a winner is how
a discussion loses a document.

The index is navigation, not decoration

Nothing draws a tree from flat titles. `page_index.py` writes one as an
ordinary page, nested by title depth rather than by manifest path order —
those disagree, since on disk `Top/System.md` sorts before
`Top/Ideas/Scale.md` while in the hierarchy System is a child and Scale a
grandchild. A parent with no page of its own still gets a node, so its
children are not hidden.

Links use `sub_url` when there is one and Gitea's `[[Title|label]]`
syntax when there is not, so the order is push, rebuild, push.

The same stances as the issue store, for the same reasons

Pull overwrites, push is additive and never deletes, change detection is
one hash and there is no drift model. A page with no `sub_url` has never
been published, and that is a durable state.

Issues gain a `wiki:` field holding page titles — titles, not URLs, so
the reference stays in the domain. It already round-trips as a foreign
key; this documents it.

Verified against a live Gitea 1.26.1: create, update with a message,
unchanged-skip, prefix-filtered pull, byte-identical round trip, and the
per-page revision history carrying the operator's own words. The probe
pages were deleted afterwards.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

skills/page/scripts/page_index.py

@@ -0,0 +1,87 @@
+#!/usr/bin/env python3
+"""
+page_index.py — write a table-of-contents page into a space. Offline.
+
+The wiki this feeds is flat: a title like `Simple Chains/Ideas/Chain core` has
+hierarchy in its name and nowhere else, and Gitea will not draw you a tree from
+it. An index page is therefore not a nicety, it is the navigation.
+
+Written as an ordinary page in the space, so it is pushed by the same command
+as everything else and needs no special case anywhere downstream. Links are
+written by TITLE rather than by URL — the wiki resolves those itself, and a
+link written that way survives every filename-escaping rule this layer
+deliberately refuses to model.
+
+    page_index.py --space claude-skills/tea --prefix "Simple Chains"
+        -> Simple-Chains.md, title `Simple Chains`
+
+    page_index.py --space claude-skills/tea --title Home
+        -> Home.md, title `Home`, listing the whole space
+
+Usage:
+    page_index.py [--space SPACE] [--prefix TITLE] [--title TITLE]
+                  [--dry-run] [--out DIR]
+"""
+import argparse
+import os
+import sys
+
+sys.path.insert(0, os.path.dirname(os.path.abspath(__file__)))
+import page  # noqa: E402
+
+
+def die(msg, code=1):
+    sys.stderr.write("%s: %s\n" % (os.path.basename(sys.argv[0]), msg))
+    sys.exit(code)
+
+
+def main():
+    ap = argparse.ArgumentParser()
+    ap.add_argument("--space", default="local")
+    ap.add_argument("--prefix", default="",
+                    help="index only this subtree; also the index's own title")
+    ap.add_argument("--title", help="title for the index page "
+                                    "(default: --prefix, else Home)")
+    ap.add_argument("--dry-run", action="store_true")
+    ap.add_argument("--out", help="wiki cache root (default: <repo>/tmp/wiki)")
+    a = ap.parse_args()
+
+    root = a.out or page.WIKI_ROOT
+    space_dir = page.space_root(a.space, root)
+    if not os.path.isdir(space_dir):
+        die("no such space: %s (looked in %s)" % (a.space, space_dir))
+
+    manifest = page.load_manifest(a.space, root)
+    prefix = page.sanitize_title(a.prefix) if a.prefix else ""
+    title = a.title or prefix or "Home"
+
+    body = page.render_index(manifest, prefix, heading=title)
+    relpath = page.path_for_title(title)
+
+    if a.dry_run:
+        sys.stdout.write(body)
+        print("-> %s (%s)" % (relpath, title))
+        return 0
+
+    dest = os.path.join(space_dir, relpath)
+    os.makedirs(os.path.dirname(dest), exist_ok=True)
+    with open(dest, "w", encoding="utf-8") as f:
+        f.write(body)
+
+    # Carries the entry's passthrough keys forward: rebuilding an index must
+    # update the page that is already published, never publish a second one.
+    prior = manifest["pages"].get(relpath, {})
+    e = dict(prior)
+    e.update(page.entry(title, prior.get("order")))
+    manifest["pages"][relpath] = e
+    page.save_manifest(manifest, root)
+
+    n = len(page.children_of(manifest, prefix) if prefix
+            else page.sorted_pages(manifest))
+    print("%s -> %s (%d entr%s)" % (title, relpath, n - 1,
+                                    "y" if n - 1 == 1 else "ies"))
+    return 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())