feat: export UMBEL_RESOLVED_DIR + UMBEL_BUNDLE_VERSION on the bundle run path

[vessux]

Summary

umbel run <bundle> now exports two vars into claude's spawn env so downstream tools can read the running bundle's identity directly, instead of scraping the --plugin-dir entry off PATH (a fragile dependency on Claude Code's plugin→PATH behavior):

• UMBEL_RESOLVED_DIR — the resolved cache dir (<cacheRoot>/bundles/<name>-<hash>).
• UMBEL_BUNDLE_VERSION — the bundle version (0.0.0+<hash>).

To guarantee the version can't drift from what the compiler writes, compile() now returns { cacheDir, version }: the 0.0.0+<hash> string is computed once and used both for plugin.json and the return value (no re-derivation, no second literal). exec.ts destructures it straight into spawnEnv. The vanilla path (execVanilla) is untouched and sets neither var.

Concrete first consumer: a dotfiles ccstatusline widget showing name@version of the active bundle.

Changes

• src/bundle/compile.ts — compile() → CompileResult { cacheDir, version }; single source for the version string.
• src/bundle/exec.ts — set UMBEL_RESOLVED_DIR + UMBEL_BUNDLE_VERSION on the bundle-resolved path.
• src/run.ts — buildBundle uses .cacheDir.
• src/args.ts — both vars documented in the --help Env section.

Test plan

• New unit test asserts the bundle-path spawn env (exec.test.ts), incl. UMBEL_BUNDLE_VERSION == compiled plugin.json version.
• New run.vanilla-env.test.ts proves the vanilla path sets neither var (scoped spawnSync mock).
• Help-text assertion in args.test.ts.
• compile.test.ts updated mechanically for the new return shape.
• Full suite 351 pass, tsc --noEmit clean, biome check . clean.
• End-to-end: built CLI + fake claude confirms UMBEL_RESOLVED_DIR = on-disk dir and UMBEL_BUNDLE_VERSION = plugin.json version.

Out of scope (captured as follow-ups)

• The issue's acceptance loosely says the var equals "the version field in bundle.md", but bundle.md frontmatter carries a bare hash field — the 0.0.0+<hash> version lives in plugin.json (the cited compile.ts line). Implemented to the cited value/line; the bundle.md-vs-plugin.json divergence is filed for discovery.

Closes #17