feat: session-freshness re-anchor skill (B16) — boundary definition + placement

[kyle-sexton]

Problem

Long-lived and resumed sessions act on stale premises: main has moved, PRs referenced in a
handoff have merged or changed shape, skills have been renamed, plugins have shipped new
versions, and locked plans reference in-flight work that has since landed. Today each session
re-derives freshness ad hoc (manual rebase checks, gh pr view sweeps, grep for renamed
surfaces); nothing owns the "re-anchor this session's assumptions against live reality" pass.

Ledger decision B16 (babysit-prs migration, docs/topics/babysit-prs-migration/PLAN.md
Brief AC7) sequences this skill last in that migration: issue first with the boundary
definition, implementation after user sign-off.

Proposed boundary (draft — for discussion)

The skill re-anchors a SESSION's working assumptions. It does NOT own:

• source-control:worktree status — per-worktree/branch staleness inventory (behind-base
  counts, PR linkage, dirty state) stays there; the freshness skill may CITE it as an input.
• source-control:babysit-prs rescan — per-PR fresh-evidence discipline (GitHub is the
  state store; every finding re-verified against the live API) stays in the babysit loop; the
  freshness skill never classifies PR feedback.

Candidate in-scope checks: handoff/PLAN premise verification (referenced PRs/issues/branches
still in the stated state), base-branch drift vs the working branch, renamed/retired skill and
plugin surfaces cited by the session's inputs, plugin-version drift between installed and
repo-source versions, memory-tier files whose subjects have since merged.

USER-RESERVED (do not implement until signed off)

• Name (verb grammar per docs/conventions naming contract)
• Placement — the re-anchor plugin now exists (0.2.0, PR feat(re-anchor): add script-the-deterministic-work corrector (0.3.0) #293) and is the natural
  placement candidate; alternatives: session-flow (adjacent to handoff/resume) or a
  standalone plugin.
• Final scan-boundary list (which candidate checks above make the cut).

Acceptance shape (post-sign-off)

Boundary doc + skill with routing evals distinguishing it from worktree status and
babysit-prs; no overlap copy — cross-cites only.

No linked issue