feat(to-plan): add sequential plan skill

Add a user-invoked `to-plan` skill — the sequential, HITL sibling of
`to-issues`. Both slice work into tracer-bullet vertical slices, but
`to-plan` produces one ordered sequence you drive by hand, one phase per
fresh context, rather than parallel independently-grabbable issues.

It publishes to the tracker `/setup-matt-pocock-skills` configured, shaped
to it: a single sequential `plan.md` for a local tracker, or a parent
issue with ordered sub-issues (native sub-issues where supported) for a
real one.

Registers the skill everywhere CLAUDE.md requires (plugin.json, top-level
and bucket READMEs, docs page), updates the `ask-matt` router's main-flow
fork to offer both to-issues and to-plan, and adds a changeset.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.claude-plugin/plugin.json

@@ -9,6 +9,7 @@
     "./skills/engineering/setup-matt-pocock-skills",
     "./skills/engineering/tdd",
     "./skills/engineering/to-issues",
+    "./skills/engineering/to-plan",
     "./skills/engineering/to-prd",
     "./skills/engineering/implement",
     "./skills/engineering/prototype",