feat: drop the kettle plugin; the binary writes its own skills

The plugin and the binary shipped on two release cadences and nothing on an
operator's machine ever checked that the one they installed described the other.
The generated flag block existed precisely so a renamed flag could not ship with
documentation recommending the old one — and then shipped one version behind the
registry it came from, which is the same bug one hop downstream.

So the prose moved into the binary. `internal/scaffold` embeds every document;
`kettle init` and `kettle gen scaffold` write them into a project's own
`.claude/`. The two cannot disagree because there is one artefact.

The namespace survived the move. A project's skills are flat, so the prefix is
spelled into the directory name (`kettle-issue`); a project's *commands* take
their namespace from a subdirectory, so `commands/kettle/init.md` is still
`/kettle:init`. Four of the six command files are thin pointers at a skill, and
that is what kept ~1,600 lines of `/kettle:…` cross-references true without a
rewrite. `init` and `auth` lost `disable-model-invocation: true` — being a
command is that property — and `auth` now restricts `allowed-tools` so a model
cannot reach `kettle auth add` at all.

`gen scaffold` writes files whole rather than splicing a region. The old
refusal protected somebody's hand-written prose around the block; that prose is
embedded now, so there is none to protect, and preserving local edits would
freeze a project's documentation at whatever version first initialized it.
`--check` warns before an upgrade discards one.

The plugin's `agents-sync.sh` — 141 lines of Python behind a filename that said
`.sh` — became `internal/mirror` and `kettle mirror`. Same seven branches, same
refusal to merge two real files that differ, now with a table test per branch
and a check that a repair converges in one pass. `--hook` is the PreToolUse
form and exits 0 on every path including a panic. It is opt-in per project,
which is strictly narrower than the plugin hook that was on for everybody who
installed it.

`kettle init --interactive` walks a person through the login, the token (read
with the echo off, so it lands in no history and no file), the repository, the
`.claude/` tree and the mirror hook. It refuses a stdin that is not a terminal
and names the flags instead: every question it asks has one, and it performs
nothing itself, so an interactive run and a flag run are one code path.

Two rules that used to be prose are now the binary's: init refuses a linked
worktree and names the main checkout, and writing into an existing
`.claude/settings.json` is refused with the snippet printed rather than
reformatting a file the operator commits.

The scaffold version stamp went to its own `.kettle/scaffold.yaml` rather than
into `config.yaml`, because unknown keys there are a hard error and that file
may be committed and read by whatever build each machine has.

golang.org/x/term becomes a direct dependency; it was already in the tree
indirectly, so no module was added.

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

cli/internal/config/AGENTS.md

@@ -1,11 +1,12 @@
 # AGENTS.md — internal/config
 
-**Two files: what this project is, and who this machine is.** The only package in
-the tree that imports yaml.
+**Three files: what this project is, who this machine is, and what was last
+written into the project's agent-harness tree.** The only package in the tree
+that imports yaml.
 
 | file | what is in it |
 |---|---|
-| `config.go` | `Project` and `Logins` (the two files), `Resolve`/`ResolveOutsideAProject`/`Require`, `Resolved` with `Complete` and `Redacted`, the `KETTLE_*` overrides, and the 0600 write |
+| `config.go` | `Project`, `Logins` and `Scaffold` (the three files), `Resolve`/`ResolveOutsideAProject`/`Require`, `Resolved` with `Complete` and `Redacted`, the `KETTLE_*` overrides, and the 0600 write |
 
 ## The split is the whole design
 
@@ -15,8 +16,18 @@ the tree that imports yaml.
 
 ~/.config/kettle/logins.yaml      logins: [{name, url, user, scopes, token}]
                                                                       mode 0600
+
+<project>/.kettle/scaffold.yaml   version: v1.4.0         a note, load-bearing
+                                  out:     .claude        for nothing
 ```
 
+The third file is the newest and the least important, which is exactly why it is
+a file of its own — see *Unknown keys are an error* below. It records what
+`kettle gen scaffold` last wrote into the project and which build wrote it, so
+`kettle config` can say when a project's skills are four releases behind the
+binary reading them. **Nothing resolves from it.** Delete it and you lose the
+warning and nothing else.
+
 `user` and `scopes` are **documentation and nothing else** — nothing is checked
 against either, and no request is refused because of one. `scopes` is what the
 token was minted with, as Gitea spells it (`write:issue`, `write:repository`),
@@ -103,17 +114,23 @@ operator upgrades; it would **not** be acceptable for `config.yaml`, which is
 committed and read by whatever version each machine happens to have. Adding a
 field to the project file means answering that first, out loud, here.
 
+`scaffold.yaml` exists because that answer came back "no". Recording which build
+wrote a project's `.claude/` tree wanted two keys, and putting them in
+`config.yaml` would have made every older `kettle` in the world stop reading a
+committed file. A separate file written and read by one binary about one
+directory carries the same cost for nothing: an older build never opens it.
+
 ## What does not belong here
 
-A request, a store path, an issue. This package reads and writes two files and
+A request, a store path, an issue. This package reads and writes three files and
 answers "who am I and where am I pointed"; [`gitea`](../gitea/AGENTS.md) takes the
 answer and dials, and the paths themselves come from
 [`project`](../project/AGENTS.md).
 
 ## Keeping this file true
 
-- **Scope:** `config.go` — the two files, their fields, the overrides, the file modes.
-- **Update it when** a field is added to either file (both tables above are the
+- **Scope:** `config.go` — the three files, their fields, the overrides, the file modes.
+- **Update it when** a field is added to any of them (the block above is the
   contract), an override is added or renamed, the location or mode of the login file
   changes, or the unknown-key policy changes.
 - **Do not** move a credential into the project file, and if that ever changes, the