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/AGENTS.md

@@ -1,6 +1,6 @@
 # AGENTS.md — internal/, and the boundaries between the packages in it
 
-Seven packages, one direction of knowledge. The diagram is in
+Nine packages, one direction of knowledge. The diagram is in
 [`cli/AGENTS.md`](../AGENTS.md); **this file owns the rules that hold it and the
 tests that fail when one is broken.** Each package's own document owns what is
 inside it.
@@ -12,7 +12,7 @@ number, a login, an HTTP call, a label colour — that shows up in
 section, an acceptance criterion, a type taxonomy — that shows up in
 [`gitea`](gitea/AGENTS.md) is in the wrong place too.
 
-## Four rules, seven tests
+## Six rules, eleven tests
 
 Each test fails on a real mistake rather than on a naming convention.
 
@@ -22,10 +22,24 @@ Each test fails on a real mistake rather than on a naming convention.
 | [`wire`](wire/AGENTS.md) imports **only** the standard library | `TestWireDependsOnNothing` and `TestWireReachesNeitherTheNetworkNorTheDisk`, the same two checks |
 | [`gitea`](gitea/AGENTS.md) must not import [`issue`](issue/AGENTS.md) **or** [`mapping`](mapping/AGENTS.md) | `TestTransportDoesNotImportTheDomain` — the transport knows numbers, logins, HTTP and JSON, and none of what they mean |
 | [`mapping`](mapping/AGENTS.md) reaches for nothing but the domain, `wire` and the SDK, and does no I/O | `TestTheBridgeTranslatesAndNothingElse` on its **direct** imports, with `os`, `net/http`, `internal/gitea`, `internal/config` and `internal/project` named; `TestTheBridgeHasNoClock` greps its sources for `time.Now` |
+| [`mirror`](mirror/AGENTS.md) imports **only** the standard library, and shells out to nothing | `TestMirrorDependsOnNothing` and `TestMirrorNeitherDialsNorShellsOut` |
+| [`scaffold`](scaffold/AGENTS.md) imports **only** the standard library, and reads nothing off the disk | `TestScaffoldDependsOnNothing` and `TestScaffoldReadsNothingOffTheDisk` |
 
 The domain's two tests were **untouched by the migration to the Gitea SDK, and
 that is the point: the domain did not notice it happened.**
 
+The last two rows are the newest and are there for the same reason as `wire`'s,
+turned outward rather than inward. `mirror` walks any directory on the machine
+and `scaffold` hands out documents that must exist wherever the binary does — so
+one import of `internal/config` would make `mirror` unusable outside a project,
+and one `os.ReadFile` would make `scaffold`'s documents files that can be
+missing. Neither failure would show up in this repository; both would show up on
+somebody else's machine.
+
+`mirror` bans `os/exec` by name, and that one is a small monument: this package
+was 141 lines of Python behind a filename that said `.sh`, so the shell-out it
+must not grow is the exact thing it used to be.
+
 ## The one rule that got weaker, and why the trade was taken
 
 The payload shapes used to live in `wire`, a package that imported the standard
@@ -76,8 +90,8 @@ and add its rule to the table above.
 
 ## Keeping this file true
 
-- **Scope:** the boundaries *between* the packages under `internal/` — the four
-  rules, the seven tests that hold them, and the history of the one that changed.
+- **Scope:** the boundaries *between* the packages under `internal/` — the six
+  rules, the eleven tests that hold them, and the history of the one that changed.
   Files: every `layering_test.go`, plus `TestTransportDoesNotImportTheDomain` in
   `gitea/client_test.go`.
 - **Update it when** a layering test is added, renamed, removed or weakened; when