fix(hub): show the Wiki migration command in Health when migration is required

[M4Y4NK-V3RM4]

What

Hub Health now tells you what to do when the Wiki index is migration_required, instead of showing "No repair recommended" with no action. This is the first fix listed in #268. The other three (population prompt wording, launchHub routing to finalize, a migration job) are not included, and some of them overlap the setup work in #271.

• WikiHealthDetails gets an optional, nullable, bounded recoveryCommand: { label, command } field (strict, max 200 chars each). Older payloads without it still parse.
• projectWikiHealthStatus sets it only for migration_required, to "Preview the required Wiki migration" / mex wiki migrate --dry-run. Every other state gets null. allowedWikiOperations and recommendedWikiOperation are unchanged, so the Hub still never runs the migration itself (explicit-mutation rule).
• The migration_required health summary now says to preview with mex wiki migrate --dry-run and then apply it with the CLI.
• HealthPage → WikiHealthReadout: when a recovery command is present and no job is recommended, the pill reads "CLI action required" and the label and command are shown in a <code> block under the facts.
• The label and command live in one small constant (src/wiki/migration/recovery.ts). mex capabilities and the Specs recovery in team/specs/cli/projections.ts now build their existing --json strings from it, so all three can't drift apart. Their output is byte-for-byte unchanged.

Why

Part of #268. A scaffold committed before finalization leaves every clone at migration_required, and Health was a dead end, even though the CLI already knows the next step (src/capabilities.ts, Specs recovery).

Type of change

• Bug fix
• New feature
• Refactor
• Docs
• CI/Tooling

How to test

1. Open the Hub on a repo whose Wiki index is migration_required (repro steps in A setup committed before finalization leaves every clone's Wiki at migration_required, with no Hub recovery #268, or delete .mex/wiki.db on a scaffold whose .mex/*.md files have no mex: key).
2. Health → Wiki index shows "CLI action required" with "Preview the required Wiki migration" and mex wiki migrate --dry-run.
3. Any other state (fresh, stale, missing…) looks exactly as before.

Run locally on top of main (54f7d7a):

• npm run typecheck (contracts, root, hub-web): pass
• npx vitest run src/hub/__tests__/services.test.ts packages/hub-contracts/src/index.test.ts src/team/specs/cli/__tests__/commands.test.ts test/capabilities.test.ts test/wiki-architecture.test.ts src/hub/http/__tests__/app.test.ts: 6 files, 141 tests pass
• cd packages/hub-web && npx vitest run src/app/page-states.test.tsx: 55 tests pass

New tests: a services test (recovery command and summary for migration_required, null for fresh), a contracts test (optional, nullable, bounded, strict), and a Health page test (command shown, "No repair recommended" absent).

Not run: the full npm test suite and npm run build. I didn't add a CHANGELOG line, to avoid conflicting with the 0.8.4 release PR. Happy to add one if you'd like.

Checklist

• Tests pass (npm test). Only the targeted files above were run.
• No breaking changes (the new contract field is optional)
• Tested locally with a real project. Not tested against a live Hub, only through unit and page tests.

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Assisted-by: AI agent (Hark) with ZCode/GLM-5.3-Flash. I reviewed the change and the test results above.