feat(fork): map fork features and report upstream overlap

[lukemaj]

Upstream can add a capability elsewhere without producing a rebase conflict. This change backfills the fork's nine feature groups and reports possible overlap before an absorption, so the absorption PR records whether to keep ours, adopt upstream and delete ours, or merge both.

The feature map is the single source for metadata and exact ownership of every allowlisted file. A dependency-free helper validates it in fork-check and scans upstream commit titles, touched paths and diffs. Shared files participate in every relevant feature's report. The report is a heuristic; decisions remain a required human review step.

Backfilled: direction, glossary, Chromeria branding, child threads, fork maintenance and CI, thread scope, Agents panel, Prism toolkit and role kits, and Prism settings.

Validation on 446f35d35ff3bdbfdad01680bcf05736281b85b8: fork-check, 12 real Git fixture tests, repository typecheck, lint, formatting and knip:check all passed. Independent Luna max review found no blockers; fork CI passed. Proof and review record. No running app, user data, deployment or upstream absorption changes. No version-policy adoption; this fork has no .version-policy.json.

Closes #20.

Implemented with GPT-6 in Codex; independent review uses Luna max in OpenCode under Model Router policy.

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.5 KiB	13.5 KiB	+2 B (+0.0%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.1 KiB	7.1 KiB	+2 B (+0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.4 KiB	6.4 KiB	0 B (0.0%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	56.2 KiB	56.2 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	9	9	0 (0.0%)	21	✅
Claude	Total thread wire	13.5 KiB	13.5 KiB	+15 B (+0.1%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.1 KiB	7.1 KiB	+5 B (+0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.4 KiB	6.4 KiB	+10 B (+0.2%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.0 KiB	57.0 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	9	9	0 (0.0%)	21	✅

Baseline: 1ab79b5 · PR result: 446f35d · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 113.9 KiB
• Claude decoded thread snapshot: 114.6 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.