docs(fork): add FORK.md upstream-merge friction map

[zephg]

What

Adds FORK.md — a single reference for how this fork diverges from upstream and which code paths will rub on each sync — and trims AGENTS.md's "Local fork status" to a pointer.

Why here

AGENTS.md is a file upstream actively edits (3 upstream commits touched it in recent history) and our fork already carries a 36-line diff against upstream's copy — a poor host for a growing fork registry that will itself conflict on every merge. FORK.md is a file upstream doesn't have, so it never produces merge markers.

Contents of FORK.md

• Divergence at a glance — the 5 local features mapped to origin commits.
• Conflict-prone code paths — per-feature tables keyed by the actual fork file changes (git log --no-merges upstream/main..local/daily-driver), each rated on two axes: upstream churn (6-month commit counts per file, real data) × change nature (isolated additive vs. shared-logic). This keeps raw churn from overselling ClineProvider.ts (81 commits, but additive) and underselling reasoning.ts (5 commits, but the semantic keystone).
• Fork-only files — added files that never conflict textually but need re-wiring if upstream restructures.
• Recurring/expected conflicts — release mechanics (version, CHANGELOG, READMEs, locales).
• Post-merge checklist — relocated from AGENTS.md so nothing is lost: pnpm vsix --force rebuild, the 22-failure test baseline, the effort-payload guard.
• Regenerate this map — the exact commands used to build it.

AGENTS.md change

"Local fork status" trimmed to a short pointer at FORK.md. The operational nuggets (VSIX rebuild, 22-failure baseline, per-provider reasoning guards) moved into FORK.md — not dropped. ## Conventions and later sections untouched.

Notes

• Docs-only; no runtime surface to verify. Sanity-checked that every code path cited in FORK.md exists in the tree.
• Base is local/daily-driver (this fork's trunk), not main.
• Known limitation, documented in the file: the map is built from non-merge fork commits, so conflicts already resolved inside past merge commits aren't captured — add a row when a new area starts conflicting.