fix(managed-skills): unsupported rows on every path; failed discovery is degraded

[Juliusolsson05]

Follow-up to #1017's internal review (ledger T10).

Why

1. The unsupported rows vanish. bug(managed-skills): provider declaring personalAgentSkills unsupported blocks every session spawn #1014 made unsupported providers informational: the unsupported:<provider> rows show in Settings instead of blocking. Those rows were appended by only two of the roughly fifteen code paths that set target statuses. The startup audit, a target conflict on enable, a disable, and the custom and installed reconciles all replaced the list without them. So Settings stopped naming the unsupported provider as soon as anything else happened.
2. A failed discovery reads "unsupported". When provider target discovery fails, resolveTargetsSafely marked conventions and custom skills with the resolution error, but not installed skills. They kept stale rows, and installedHealth checked "zero targets → unsupported" before any error. The skill therefore said "no provider can take this" when the truth was "we could not look".

Change

• withUnsupportedRows adds the rows when a snapshot is built, derived from this.targets, so every path agrees.
  • It replaces the three near-identical builders (conventions, custom, installed).
  • Health still reads the stored statuses, which never contain these rows.
• Installed skills get the resolution error row on a discovery failure.
• installedHealth checks error before "unsupported", as health() and customHealth() already do.
• Two Grok nits from ledger T6: an import spacing fix, and a doubled word.

Tests (the real service, fail-first)

• AgentCodeConventionsService.system.test.ts:
  • the first snapshot after startup names unsupported:grok;
  • a target conflict on enable keeps that row beside the conflict row.
• AgentCodeInstalledSkillsService.system.test.ts: after an install, a discovery failure, then audit(), the skill is degraded with the provider-target-resolution error row.

All three fail on main. The managed-skills system and unit suites pass (128), the settings renderer suites pass (18), and npx tsc -b is clean.

🤖 Generated with Claude Code

[Juliusolsson05]

Review (CHANGES REQUESTED) addressed in c1dd4882:

1. MEDIUM: the discovery-error row is no longer a delete blocker. It names no file and has no fingerprint, so it could never be approved. The stillOwnedKeys journal check still prevents forgetting a skill whose files are on disk. The fail-first test (install, disable, discovery fails, audit, delete) fails with the old blocker filter.
2. LOW: an unsupported row in the custom-skills list shows its provider.
3. LOW: the stale append comments are rewritten.
4. NIT: an installed skill's error row shows its message.

Managed-skills and settings: 147/147, and tsc is clean.