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/project/AGENTS.md

@@ -9,7 +9,7 @@ one in the tree with no other package below it.
 
 | file | what is in it |
 |---|---|
-| `project.go` | `Marker`, `Anchors`, `Parents`, `GitDirOf`, `MainWorktree`, `Root`, and the paths resolved from it — `StoreRoot`, `PayloadRoot`, `ConfigPath` — plus `NotFoundError` |
+| `project.go` | `Marker`, `Anchors`, `Parents`, `GitDirOf`, `MainWorktree`, `Root`, and the paths resolved from it — `StoreRoot`, `PayloadRoot`, `ConfigPath`, `ScaffoldPath` — plus `NotFoundError` |
 | `init.go` | `Init`: creates the marker, migrates an older layout in, gitignores `.kettle/`. `ClashError` is its refusal |
 | `project_test.go` | the walk, including the worktree hop and the "no marker anywhere" answer |
 
@@ -87,6 +87,12 @@ A non-empty `start` overrides both anchors and exists so resolution can be
 exercised against a scratch tree — which is what the test suite does, and why
 every fixture also strips `CLAUDE_PROJECT_DIR`.
 
+`MainWorktree` has a second caller now, and it is the one that made the function
+worth having in public: `kettle init` refuses to run where it answers, and names
+the main checkout instead. That rule used to be a paragraph in a skill somebody
+had to read, which stopped being good enough the moment an interactive wizard
+became the front door — a front door cannot assume anybody read anything.
+
 ## What does not belong here
 
 Anything that reads or writes an issue, a config file or a socket. This package

cli/internal/project/project.go

@@ -29,9 +29,10 @@ const Marker = ".kettle"
 // Everything under the marker, each resolved by the same walk so that which
 // command wrote a file cannot change where it landed.
 var (
-	storeParts   = []string{Marker, "issues"}
-	payloadParts = []string{Marker, "payload"}
-	configParts  = []string{Marker, "config.yaml"}
+	storeParts    = []string{Marker, "issues"}
+	payloadParts  = []string{Marker, "payload"}
+	configParts   = []string{Marker, "config.yaml"}
+	scaffoldParts = []string{Marker, "scaffold.yaml"}
 )
 
 // Anchors are the directories a root search starts from, in order, first hit
@@ -200,6 +201,17 @@ func PayloadRoot(start string) string { return under(start, payloadParts) }
 // ConfigPath is the absolute path of the project's tracker config, or "".
 func ConfigPath(start string) string { return under(start, configParts) }
 
+// ScaffoldPath is the absolute path of the record of what was written into the
+// project's `.claude/` tree, or "".
+//
+// A file of its own rather than a field in config.yaml, and the reason is a rule
+// stated in [config]: an unknown key in config.yaml is an error rather than a
+// silent drop, so a field added there is a one-way door for a file that may be
+// committed and read by whatever version each machine happens to have. This
+// record is written and read by one binary about one directory, so it can carry
+// that cost where the shared file cannot.
+func ScaffoldPath(start string) string { return under(start, scaffoldParts) }
+
 func under(start string, parts []string) string {
 	root := Root(start)
 	if root == "" {