fix(strategy): warn when a commit is left unlinked with a live sibling-worktree session

[ColeDriver]

What & why

Fixes the silent no-trailer case in #1852. Commits made in a worktree with no matching session were silently left without an Entire-Checkpoint trailer — even when a live agent session ran in another worktree of the same repo. In one reported case this unlinked 101 of 108 commits (94%). The miss was logged at DEBUG, so nothing surfaced it to the person running git commit.

Change

On the no-session path in prepare-commit-msg, the hook now checks whether a live agent session exists in a different worktree of the same repo. When one does, it (1) prints a notice to stderr during the commit — the same channel warnIfAttributionDiverged uses, so the failure is actually visible — and (2) keeps a structured WARN in .entire/logs for diagnostics. The stderr notice reads:

entire: this commit was not linked to a checkpoint (live agent session in ../feature).
  run 'entire session adopt --from ../feature --yes' to link this worktree.

Sessionless commits — the common case — stay quiet; the check is skipped when listing errored. Underlying matching is unchanged — this makes the residual silent case visible, it doesn't change what gets linked.

Notes / decisions

• "Live" = non-ended AND recently interacted (24h activeSessionInteractionThreshold), mirroring the staleness guard used elsewhere in this file — a crashed session that never reaches ENDED won't nag forever. ENDED / stale / adopted-away / imported don't count.
• --yes is required in the hint: same-repo adoption routes through adoptFromSameSessionStore, which needs it even on first adoption — the bare command would always error.

⚠️ Tradeoff — maintainer call before merge

1. Overlaps in-flight matching work (use github runners #1/Move "clean" root command to "session cleanup", some refactoring #2 in the issue). That fix will shrink when this warning fires, possibly to just the ambiguous multi-worktree case it can't resolve. Whoever owns matching should confirm they want the interim warning now vs. folding it in later.
2. Prints on every affected commit (each is a real, separate miss). Repo has throttle mechanisms — happy to rate-limit / show-once-per-session if preferred.

Tests

• Table-driven unit tests (t.Parallel()): isLiveSessionInOtherWorktree (same-worktree / sibling-active / sibling-idle / stale / never-interacted / ended / adopted-away / imported), uniqueWorktreePaths, formatUnlinkedCommitNotice.
• Full strategy suite passes; maintidx + golangci-lint v2.11.3 clean on changed files; go build ./..., go vet, gofmt clean.

Refs #1852