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/config.go

@@ -47,6 +47,14 @@ const projectHeader = `# kettle — project configuration
 # Overrides, when you need one: ` + EnvLogin + `, ` + EnvRepo + `, ` + EnvURL + `, ` + EnvToken + `.
 `
 
+const scaffoldHeader = `# kettle — what was last written into this project's agent-harness tree.
+#
+# Written by ` + "`kettle init`" + ` and ` + "`kettle gen scaffold`" + `; read by
+# ` + "`kettle config`" + `, which says so when the build that wrote the tree is not the
+# build that is installed now. Nothing resolves from this file — deleting it
+# costs the warning and nothing else.
+`
+
 // Project is `<project>/.kettle/config.yaml`.
 type Project struct {
 	// Login names an entry in the machine-wide login file. Never a token.
@@ -55,6 +63,27 @@ type Project struct {
 	Repo string `yaml:"repo"`
 }
 
+// Scaffold is `<project>/.kettle/scaffold.yaml`: what `kettle` last wrote into
+// this project's agent-harness tree, and which build wrote it.
+//
+// A file of its own rather than two more keys in Project, and that is the rule
+// in "Unknown keys are an error" being obeyed rather than worked around: a field
+// added to config.yaml is a one-way door for a file that may be committed and
+// read by whatever version each machine happens to have. This one is written and
+// read by a single binary about a single directory, so it can carry that cost.
+//
+// It exists to answer one question — "were these documents written by the kettle
+// that is installed now?" — because the documents are generated whole and an
+// operator has no other way to tell a current tree from one four releases old.
+type Scaffold struct {
+	// Version is what `kettle version` reported when the tree was written. A
+	// hand build says `dev` and means it.
+	Version string `yaml:"version"`
+	// Out is where the tree went, relative to the project root when it is
+	// underneath it.
+	Out string `yaml:"out"`
+}
+
 // Login is one set of credentials for one Gitea instance.
 type Login struct {
 	Name string `yaml:"name"`
@@ -139,6 +168,43 @@ func SaveProject(path string, p *Project) error {
 	return os.WriteFile(path, append([]byte(projectHeader+"\n"), body...), 0o644)
 }
 
+// ScaffoldPath is where this project records what it last had written into it,
+// or "" with no project.
+func ScaffoldPath(start string) string { return project.ScaffoldPath(start) }
+
+// ReadScaffoldFile reads a scaffold record at a path already known, reporting
+// whether the file was there.
+//
+// A missing file is the zero value and not an error: a project initialized
+// before this record existed, or one written with --no-scaffold, has nothing to
+// say here and that is an ordinary state rather than a fault.
+func ReadScaffoldFile(path string) (*Scaffold, bool, error) {
+	raw, err := os.ReadFile(path)
+	if os.IsNotExist(err) {
+		return &Scaffold{}, false, nil
+	}
+	if err != nil {
+		return nil, false, err
+	}
+	var s Scaffold
+	if err := strictUnmarshal(raw, &s); err != nil {
+		return nil, true, fmt.Errorf("%s: %w", path, err)
+	}
+	return &s, true, nil
+}
+
+// SaveScaffoldFile writes the scaffold record.
+func SaveScaffoldFile(path string, s *Scaffold) error {
+	body, err := yaml.Marshal(s)
+	if err != nil {
+		return err
+	}
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		return err
+	}
+	return os.WriteFile(path, append([]byte(scaffoldHeader+"\n"), body...), 0o644)
+}
+
 // LoginsPath is the machine-wide login file.
 //
 // One file per machine, deliberately outside every working tree: which tokens