Plan plugin migration to metaFields + zones

Acceptance Criteria

• plugins/plan/src/config.ts rewritten. Each of the five entity runes (Work, Bug, Spec, Decision, Milestone) replaces its slots: [...entitySlots] + structure: { … } block with:

  • metaFields declaring each field's metaType, metaRank, label, condition, tag (for <time> etc.), and sentimentMap where applicable.
  • zones.eyebrow = { left: ['id'], right: ['status'] } for the primary header.
  • zones.metadata = { fields: [...] } for the descriptive header (priority, complexity, assignee, milestone, source, created, modified, tags — varies per entity type).
  • sections.title, sections.blurb, sections.body retained as today.

• entitySlots constant deleted. The slots: [...entitySlots] array is no longer needed (engine derives render order from the canonical vocabulary).

• Per-entity field shapes mirror today's behaviour.

  • Work: id, status, priority, complexity, assignee, milestone, source, created, modified, tags. Eyebrow: id+status. Metadata: the rest.
  • Bug: id, status, severity, assignee, milestone, source, created, modified, tags. Eyebrow: id+status. Metadata: the rest.
  • Spec: id, status, version, supersedes, created, modified, tags. Eyebrow: id+status. Metadata: the rest.
  • Decision: id, status, date, supersedes, source, created, modified, tags. Eyebrow: id+status. Metadata: the rest.
  • Milestone: name (used as id), status, date, target, done / total counts (from WORK-281 rollup), created, modified, tags. Eyebrow: name+status. Metadata: the rest.
  • sentimentMap values preserved verbatim from today's config.

• plugins/plan/src/index.ts theme.runes entry updated. The exported theme config for the plugin reflects the new shape. Existing per-rune CSS (work.css, bug.css, etc.) inspected for selectors that referenced the old class names (__header-primary, __header-secondary); rewritten to the new zone-named selectors (__eyebrow, __metadata).

• Rune-specific CSS quirks preserved. Complexity dots on work ([data-complexity="…"] > .rf-work__metadata > .rf-work__complexity dd::after) continue to render correctly under the new DOM. Assignee @ prefix preserved. Body section dividers untouched.

• Plan-site rendering passes. All existing plan-site snapshot tests in packages/content/test/plan-site-dogfood-real.test.ts pass with the new DOM shape (zone-named classes, def-list metadata). Visual QA on a representative work / bug / decision / spec / milestone page in the rendered plan site.

• Backwards-compat shim NOT exercised by plan plugin itself. Plan plugin is fully migrated; the shim only catches third-party consumers (which the plan plugin is not). Verify via the shim's first-encounter warning — it should not fire for any plan rune.

• Plan plugin tests updated. Tests in plugins/plan/test/ that snapshot rune output reflect the new DOM. Scanner tests untouched (they read frontmatter / rune attributes, not output DOM).

• Docs. Plan-rune doc pages (site/content/runes/plan/{spec,work,bug,decision,milestone}.md) updated:

  • Output contract snippets reflect the new DOM (zone-named classes, def-list metadata).
  • Any examples referencing __header-primary etc. updated to __eyebrow / __metadata.