refactor!: rewire the plugin onto the kettle binary, and rename it

BREAKING: the plugin is `kettle`, not `tea`, and its commands are `/kettle:*`.
It also now needs a binary on PATH that it did not need before; the README and
every skill say how to get one and what a missing one looks like.

The plugin was 3800 lines of Python doing what a compiled binary does better,
and the name pointed at a tool that no longer takes part: `tea` is Gitea's CLI,
and since the transport moved into the binary nothing here shells out to it for
issues at all. A plugin named after it was going to keep suggesting otherwise.

Deleted: 19 scripts, the 14-file unittest suite, and the tea-guard hook. The
guard blocked any `tea` invocation that would run under a login the model picked
instead of the operator; the binary holds its own credentials and reads the
pinned login out of the project's own config, so that failure is no longer
expressible and there is nothing left to police. agents-sync stays — it is about
AGENTS.md symlinks and has nothing to do with any of this.

What the plugin keeps is what only a plugin can carry: the rules an operator
states and a binary cannot enforce. `init` still refuses to run inside a linked
worktree and still may not be model-invoked, because which directory is the
project is a statement a person makes. The issue format reference stays here and
stays the source of truth. The runner subagent is still for batches and still
may not decide what an issue says.

The command reference in the issue, sync and project skills is GENERATED from
the binary's own command registry, between markers, so a flag that changed
cannot ship with a skill that recommends the old one. `kettle gen skills
--check` exits non-zero when they drift. The generator owns the region and
nothing outside it: the frontmatter description, which is what decides whether a
skill loads at all, stays hand-written.

`use` survives and is the one place `tea` is still named — for releases,
webhooks and actions, which kettle does not cover. Its instruction to write
`--login "$GITEA_LOGIN"` and let the hook substitute the pin was true until this
commit and is now rewritten: `tea` keeps its own configuration, kettle keeps
its own, and configuring one configures nothing in the other.

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

cli/internal/cmd/init.go

@@ -76,11 +76,11 @@ one would write issues into whatever it happened to be installed in.
 machine, outside every working tree, managed with ` + "`kettle auth`" + `.
 
 All of it is idempotent: it creates .kettle/issues and .kettle/payload, migrates
-an older store in if it finds one (tmp/ or .tea/), writes the config without
-disturbing settings it was not given, and adds .kettle/ to .gitignore. Each
-migration is a move, not a copy — two stores is the state the marker exists to
-prevent — and it refuses to pick a winner when both sides hold a file of the
-same name.
+an older store in if it finds one (either layout the tea plugin used, oldest
+first), writes the config without disturbing settings it was not given, and adds
+.kettle/ to .gitignore. Each migration is a move, not a copy — two stores is the
+state the marker exists to prevent — and it refuses to pick a winner when both
+sides hold a file of the same name.
 
 Do NOT run this inside a linked worktree. A worktree is the same project on
 another branch and reaches the store by a hop out to the main checkout; a marker