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>

plugins/AGENTS.md

@@ -6,9 +6,21 @@ plugin uses stays relative to itself.
 
 | plugin | what it is |
 |---|---|
-| [`kettle/`](kettle/AGENTS.md) | issues as local markdown, driven by the `kettle` binary in [`cli/`](../cli/AGENTS.md). Skills, a runner subagent, two hooks |
 | [`tdl/`](tdl/AGENTS.md) | Three Dots Labs Go conventions as an enforceable rule set. One skill, no binary, no state |
 
+**There used to be a second one, and where it went is the useful part.** `kettle`
+was a plugin wrapping a binary, and the two shipped on separate cadences with
+nothing checking that an installed plugin described the installed binary. Its
+documents are embedded in the binary now
+([`cli/internal/scaffold`](../cli/internal/scaffold/AGENTS.md)) and `kettle init`
+writes them into a project's own `.claude/`.
+
+That is not a reason to stop writing plugins here. It is a test to apply to the
+next one: **does this plugin describe something whose version it cannot see?** If
+it does, whatever ships that thing should ship the documents too. `tdl` passes
+easily — it has no binary, no state and no network, so there is no second version
+for it to disagree with.
+
 ## Adding one
 
 Two edits, and nothing else in the repository needs to know:
@@ -49,5 +61,6 @@ what the installed plugin reports. Keep them saying the same thing.
 - **Scope:** what a plugin directory is, how the catalog points at it, and the rules
   common to all of them. The table names every plugin here.
 - **Update it when** a plugin is added or removed or renamed, or when the catalog
-  gains a field a plugin author has to fill in.
+  gains a field a plugin author has to fill in. If the last one ever goes, this
+  directory goes with it.
 - **Do not** describe an individual plugin's skills or design; each has its own file.