Add a GitHub Issues backend to Pathfinder

Chart Step 1 now asks where the map should live: local files under
gitignored specs/ (the default) or a pathfinder:map issue whose decision
questions are its sub-issues, driven by gh. The pick is recorded as the
first Ground rules bullet and never re-asked; Auto-Discovery resolves
either backend, and an issue URL routes straight to github.

On github the local model maps onto the tracker's own primitives rather
than being simulated in issue bodies: sub-issues, native issue
dependencies for blocking, the assignee as the claim, one
pathfinder:<type>-<mode> label so type and mode cannot drift, and an
Answer comment plus a close reason that distinguishes a decision from a
question ruled out of scope. Both wiring calls key on the database id,
not the number. The map body therefore carries no checklist at all.

Sketches, secrets and the PLAN-DRAFT stay on local disk regardless --
plan2code-1-plan discovers its input with ls specs/ and has no notion of
a tracker. github is only offered after a five-check preflight, and the
offer names the repo's visibility, because a map on a public tracker
publishes the destination and the codebase recon. The Step 3 recon is
held until Step 6 so the no-fog off-ramp leaves no litter.

Depth lives in a seventh reference file; the orchestrator was
recompressed to absorb the new section within the 11,000 char limit by
removing duplication with its references. Adapted from the GitHub
tracker doc behind Matt Pocock's wayfinder skill (MIT).

AI Assisted

src/plan2code-0-pathfinder-references/questions.md

@@ -1,5 +1,7 @@
 # Questions & Map Format
 > Part of plan2code-0-pathfinder — the on-disk format both modes share: directory layout, `NN` numbering, the question-file schema, `Type:` vocabulary, and the marker / blocking rules. The main file keeps only the marker legend and a layout gist; the authority is here.
+>
+> **This file describes the `local` backend only.** If `## Ground rules` says `**Backend:** github`, the equivalence table in `github-issues.md` replaces every rule below — there are no files, no `NN`, no schema lines, and no checklist.
 
 ## Layout