docs(worktree-isolation): multi-repo feature-group pattern + adoption reference

[KailasMahavarkar]

What this adds

Extends the worktree-isolation skill from single-repo isolation to a full
multi-repo feature-group pattern, and ships a standalone adoption reference so
anyone can port the logic into their own stack.

The problem it documents

A feature often spans several repos - a backend, a worker/AI service, a frontend -
each its own git repo with its own branch. Worktrees isolate one repo; nothing
binds worktrees across repos, so creating, running, tracking, and tearing down
the set is manual and easy to get wrong.

The lifecycle the docs cover

 go  ──►  new/add  ──►  env  ──►  up  ──►  st  ──►  claim-oauth  ──►  down  ──►  rm
 pick     create        seed      run      watch    OAuth :8080       stop      unbind
 set      + branch      .env      stack    drift    (singleton)                 (keep code)

Key decision: explicit registry, not branch-name matching

The load-bearing choice a reviewer cannot reconstruct from the diff:

approach	binds	verdict
exact branch match	only byte-identical branch names	misses drift (feat/x-integration vs feat/x-attribution-ui)
branch-token heuristic	fuzzy prefix/suffix strip	false positives, fragile
explicit registry	exactly what you declare	correct - chosen

Real feature branches drift per repo, so any name-based inference is unreliable.
A group / role / worktree-path / slot registry is the single source of truth.

What changed

• SKILL.md: new "Multi-Repo Feature Groups" section - registry over
  heuristics, membership lifecycle (add/remove), and running the set (port
  slots, OAuth redirect pinning, config-not-carried-by-worktree, one grouped
  session). Plus matching Quick-Reference and Red-Flag rows.
• references/multi-repo-feature-groups.md (new): tool-agnostic recipe - the
  registry schema, the port = base + 10*slot formula, run-script generation
  (env-prefix vs --port), the OAuth-claim mechanism, a config block to fill
  in, and reusable bash snippets (no-subprocess branch read, OAuth port-swap).

Verifying

Docs-only; nothing in the plugin runtime changed. The pattern is backed by a
working reference implementation (shell tooling) that was exercised end-to-end -
create, seed, run, status, OAuth-claim, teardown - against throwaway repos.