docs: document that umbel apply warms the cache (Closes #11)

[vessux]

Closes #11.

What

umbel apply <name> is documented as pin-only, but runBundleApply (src/run.ts:371-374) builds the bundle before writing the pin — warming the cache, atomically updating the by-name/<name> symlink, and printing a built <path> line alongside pinned …. The docs never said so, making the extra output + disk write read like a bug.

This is a docs-only change; apply's behavior is unchanged.

Changes

• README verb table — apply comment now reads pin … + warm cache.
• README prose — new note after the verb table explaining the build side effect, the by-name/<name> update, the built <path> output line, and that a subsequent plain umbel run launches without a rebuild. Clarifies apply --vanilla builds nothing.
• docs/bundles-spec.md — primary apply description (the "writes a single-candidate bundle pin" sentence) now leads with the build + cache-warm + by-name + built <path> behavior; CLI-surface table comment updated to build + write pin.

Acceptance criteria

• README apply entry mentions the cache-build + by-name update side effect
• The spec's apply description mentions the same
• Docs reflect the built <path> output line

Out of scope (per issue)

• No change to apply's behavior — the build side effect stays.
• The --vanilla apply path (clarified as no-build, not changed).