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

CHANGELOG.md

@@ -2,6 +2,24 @@
 
 All notable changes to Plan2Code will be documented in this file.
 
+## v2.1.0
+
+### ✨ Added
+
+- **GitHub Issues backend for Pathfinder** — `/plan2code-0-pathfinder` no longer assumes local files. Chart Step 1 now asks, HITL and never self-picked, where the map should live: **local** (the default — files under gitignored `specs/<idea>/pathfinder/`, private and solo) or **github** (a `pathfinder:map` issue whose decision questions are its sub-issues, driven by the `gh` CLI). The pick is recorded as the first `## Ground rules` bullet, never re-asked and never switched mid-map, and Auto-Discovery resolves either backend — an issue URL or number as the argument routes straight to `github`.
+
+  On `github`, the local model maps onto the tracker's own primitives rather than being simulated in issue bodies: a question is a **sub-issue** (`sub_issues` endpoint), blocking is GitHub's **native issue dependencies** (`dependencies/blocked_by`, so the frontier renders in GitHub's UI without opening the map), the claim is the **assignee**, `Type:` becomes a single `pathfinder:<type>-<mode>` label so type and mode cannot drift, `Locked: yes` becomes `pathfinder:locked`, and resolution is an `## Answer` comment followed by a close — `completed` for a decision, `not planned` for a question ruled out of scope. Both wiring calls key on the issue's **database id**, not its `#number`. The map body therefore carries no question checklist at all: the frontier is a live query, which removes the single largest source of drift in the local backend.
+
+  Three things stay on local disk whatever the backend: runnable sketches (`specs/<idea>/pathfinder/sketch-NN/`), anything secret, and the `PLAN-DRAFT-<date>.md` — `/plan2code-1-plan` discovers its input with `ls specs/` and has no notion of a tracker, so a draft that existed only as an issue would be invisible to the rest of the pipeline.
+
+  Guardrails carried over from the local backend's assumptions: `github` is only offered after a five-check preflight (`gh` present, authenticated, GitHub remote, issues enabled, push access), and the offer must name the repo's **visibility** in the same breath, because a map on a public tracker publishes the destination, the rejected alternatives, and the codebase recon. The Step 3 recon is held in-session and published at Step 6, so the Step 4 no-fog off-ramp leaves no litter on a shared tracker. A `gh` failure mid-session stops the session rather than falling back to local files, which would fork the map.
+
+  Depth lives in a seventh reference file, `src/plan2code-0-pathfinder-references/github-issues.md` — preflight, label set, the local↔GitHub equivalence table, create-then-wire charting, the frontier query, resolve and out-of-scope flows, reconcile, the trail footer, handoff, and a failure-mode table. Adapted from the GitHub tracker doc behind Matt Pocock's [`wayfinder`](https://github.com/mattpocock/skills/tree/main/skills/engineering/wayfinder) skill (MIT).
+
+### 🔧 Changed
+
+- **`src/plan2code-0-pathfinder.md` recompressed** to absorb the new `## Backend` section within the 11,000-character workflow-file limit — duplication between the orchestrator and its reference files was removed (the local layout and marker legend now live only in `questions.md`; Form A/B footer detail only in `trail.md`), and step text tightened. No behaviour was dropped.
+
 ## v2.0.0
 
 Ports upstream v1.17.0 and v2.0.0 into Plan2Code.