fix(server): clarify Forgejo conflict and validation failures

[rynfar]

Forgejo API failures currently give users generic copy even for conflict (409) and invalid-request (422) responses. This change gives those statuses specific, stable descriptions while preserving the observed HTTP status and reason for authentication, forbidden, rate-limit, and not-found failures. Error bodies are not read, so a broken body stream cannot erase a known status.

This is a partial, privacy-preserving adaptation of T3 #11593. It deliberately does not display the server's exact merge-rejection message: an untrusted Forgejo response can contain confidential text or HTML. The exact rejection reason remains unavailable from this path. The remaining web UI hunks of #11593 are outside this PR; its iOS terminal fix already landed in #770.

Validation: vp test run apps/server/src/sourceControl/SourceControlDiscovery.test.ts (27 passed); vp run --filter t3 typecheck; scoped vp lint; git diff --check. An independent adversarial review found that the initial response-body parser could leak short secrets and lose HTTP classification when its stream failed; both were removed and re-reviewed at the final head.

Model: GPT-6 Sol. Harness: Pylon.

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1 Skipped Deployment

Project	Deployment	Actions	Updated
pylon-marketing	Ignored	Preview	Sep 27, 2026 3:30am UTC

[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	14.0 KiB	14.0 KiB	−18 B (−0.1%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.3 KiB	7.3 KiB	+6 B (+0.1%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.7 KiB	6.7 KiB	−24 B (−0.3%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	58.0 KiB	58.0 KiB	0 B (0.0%)	66.4 KiB	✅
Codex	Live turn messages	9	9	0 (0.0%)	21	✅
Claude	Total thread wire	14.0 KiB	14.0 KiB	−14 B (−0.1%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.3 KiB	7.3 KiB	−1 B (−0.0%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.7 KiB	6.7 KiB	−13 B (−0.2%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	58.9 KiB	58.9 KiB	0 B (0.0%)	66.4 KiB	✅
Claude	Live turn messages	9	9	0 (0.0%)	21	✅

Baseline: 69f26bc · PR result: 0ab82f4 · 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: 115.7 KiB
• Claude decoded thread snapshot: 116.4 KiB

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