fix(registry): validate frontmatter thinkingLevel against the closed enum (#90)

[rz1989s]

Closes #90.

Problem

parseAgentFile parsed agent frontmatter thinkingLevel with a bare cast — an invalid value (thinkingLevel: "ultra", or a non-string) reached createAgentSession unvalidated and silently no-op'd, while the identical typo in fleet settings.json warns (#78 guard). Inconsistent validation for the same value class.

Design choice (the issue's micro-decision)

The issue offered "warnings-out param vs registry-load-time surface". Both miss the parser's own precedent: backend is the same closed-enum value class and already throws FrontmatterError — no warnings channel needed. discovery.ts already catches FrontmatterError → actionable warning + skip that file, siblings still load. So:

• Invalid thinkingLevel → FrontmatterError naming file, bad value, valid levels — same contract as backend, surfaced by discovery as a warning (consistent with enh: fleet.defaultSubagentThinking — fleet-wide subagent thinking-level default (split from #64) #78's "actionable warnings, never silent swallow" rule).
• YAML-empty value (thinkingLevel: → null) counts as absent — no error.

Changes

• src/registry/frontmatter.ts: THINKING_LEVELS + isThinkingLevel() move here (canonical home, next to the type — avoids a settings→registry inverted dependency); parseAgentFile validates before returning.
• src/settings/fleet-settings.ts: imports the guard from frontmatter, re-exports isThinkingLevel/THINKING_LEVELS for back-compat (zero external consumers in-repo today, but the package ships raw ts).
• README: one paragraph documenting the frontmatter validation behavior.
• Tests: +6 (invalid string throws w/ actionable message; non-string throws; absent → undefined; null → undefined; all 7 levels parse; discovery integration — bad file skipped + warned, sibling loads). 819/819 (was 813).

Verification

• pnpm typecheck ✅ standalone
• pnpm test:run ✅ 819/819 standalone
• TDD: RED confirmed first (3 new tests failed against the bare cast with "Missing expected exception")