docs: align bundle.md example with real umbel build output

[vessux]

Closes #10

What

The resolved-bundle.md example in docs/bundles-spec.md ("Self-describing bundle.md") diverged from what umbel build actually emits, in ways that could mislead machine consumers parsing bundle.md. Regenerate the example block verbatim from real output and add a note documenting the contract.

Discrepancies fixed (spec → actual)

• Key order: was name, description, skills, agents, mcps, mergeMcp, hash; now matches the emitter's name, hash, description, skills, agents, hooks, mcps[, mergeMcp][, settings] (resolvedFrontmatter in src/bundle/compile.ts).
• Qualified refs: was de-qualified (skills: [base-skill, pandas-cheatsheet]); now keeps qualified <source>/<leaf> refs as actually emitted, with an explicit note that they stay qualified (the cache dirs use canonical names; the bundle.md refs do not).
• List style: was flow [a, b]; now block-style (one - per line) — that's what yaml's stringify produces.
• hooks field: added (was omitted).
• mergeMcp / settings: documented as appearing only when set / non-empty (no longer always-shown mergeMcp: false); example now includes a settings block.
• Invocation block: single-space flag/value separation to match formatClaudeInvocation (was aligned columns).

Verification

• Frontmatter block reproduced verbatim by running the real yaml stringify over resolvedFrontmatter's output — byte-faithful.
• Invocation format cross-checked against computeClaudeArgs + formatClaudeInvocation.
• test/unit/bundle/compile.test.ts: 32/32 pass.

Scope

Docs only — the emitter is unchanged (the example follows the code, per the issue). The stale "Implementation status" section is out of scope (tracked in #8).

Note: while verifying, hit a pre-existing intermittent flake in the GC test gcBundles "does nothing special when the symlink is absent" (mtime tie under full-file runs). Not caused by this change; captured separately for discovery.