Add --issues mode: track the plan as GitHub/GitLab issues instead of PLAN.md

[atomantic]

Summary

Adds an opt-in --issues flag that lets a repo track its plan in the GitHub/GitLab issue tracker instead of PLAN.md. The default PLAN.md behavior is unchanged.

• /do:replan --issues runs the full audit/triage/prune lifecycle against labeled issues (default label plan, override with --issues-label <name>): closes done/stale issues with an evidence comment, files issues for opportunities it surfaces, and comments + drift-labels any item that would now remove a newer feature (never auto-closing a drift).
• It always reads PLAN.md when present, migrates every open item out to the tracker, and empties PLAN.md to a note pointing at the Issues page — so PLAN.md stops churning and causing merge conflicts while work happens on issues. The issue number is the stable ID (cos/<task>/issue-<n>/<agent> branches); kebab-slugs don't apply in issue mode.
• Actionable-issues invariant: before migrating an item, replan surfaces any open question/decision and prompts the human to resolve it (folding the answer into the issue body), so every filed issue is immediately claimable. A deferred decision is the only thing that may remain in PLAN.md.
• Consistent across the toolchain: a shared lib/plan-issue-mode.md partial extends the flag to every command that records plan items — /do:better, /do:better-swift, and /do:depfree file deferred findings as labeled issues instead of writing a ## … Audit section to PLAN.md; /do:review and /do:rpr file a deferred finding as an issue instead of a PLAN.md line.
• Reuses the existing gh/glab VCS-host detection; aborts if neither CLI is authenticated rather than silently writing PLAN.md.

Test plan

• npm test — all 128 tests pass (includes the install.sh/uninstall.sh LIBS allowlist snapshot, updated for the new lib/plan-issue-mode.md).
• Verified the new partial inlines for Codex/Antigravity and path-rewrites for Claude via transformCommand.
• Local review gate: fixed glab label create to pass the required --color (the || true would otherwise have silently skipped label creation on GitLab).
• Manual e2e (issue mode) is documented in commands/do/replan.md: with gh authenticated, create a plan-labeled issue, run /do:replan --issues, and confirm close/create/comment + PLAN.md emptied to the tracker note.