fix(factory): remove dead gh PR resolver fallback

[miyaontherelay]

Summary

• make the mounted GitHub projection the sole issue-to-PR resolver
• remove the production-dead gh pr list fallback, its negative backoff, and candidate parser
• retain the mounted resolver's positive cache and update the feature manifest

Why Option B

The connected App surface does not currently expose a direct PR read: GithubConnectionRead and the authenticated GithubConnectionWrite extension expose only getIssue, and the Relayfile client wires that to issue-file reads. The mount projection (listTree/readFile) is the only existing App-fed PR read path.

Production has no gh binary, so the removed fallback could only emit spawn gh ENOENT, be caught, and return no result. Keeping it obscured that the mount is load-bearing.

This PR intentionally does not touch the other gh read call sites or the write-side App identity work in #388.

Verification

• red ablation (production resolver reverted): 7 failed, 651 skipped
• green focused suite: 7 passed, 651 skipped
• full orchestrator suite: 658 passed
• build: passed
• feature map: 323 features, no advisories

The full repository run also showed four load-sensitive failures. Targeted runs on untouched origin/main reproduced the affected families; notably the fleet ownership timeout failed 7/8 times on main and 5/8 times on this branch.

[coderabbitai]

Important

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⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro Plus

Run ID: 01be873b-407b-4901-afd1-d3be63df3ea6

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[github-actions]

@coderabbitai review

Requested for exact head 1772d02cba5583c525c6172fc67a105f0d924fac.