fix: include dist/context.js in npm package + mark context as Beta

[bhvbhushan]

What broke

vibecop init --context was completely non-functional in 0.4.1 and 0.4.2. The generated hooks pointed to a non-existent file because dist/context.js was never included in the npm tarball.

Root cause: prepublishOnly ran bun run build (produces dist/cli.js) but NOT bun run build:context (produces dist/context.js). The files array in package.json listed dist/context.js but the build step never created it.

Fix

1. Added bun run build:context to prepublishOnly script
2. Marked context optimization as Beta everywhere (README, CHANGELOG, Starlight docs, CLI output)
3. Added warning to re-run init --context after upgrading vibecop

Verified

$ npm pack && tar tzf vibecop-*.tgz | grep dist/
package/dist/cli.js
package/dist/context.js     ← now included

$ npm install ./vibecop-0.4.3.tgz && npx vibecop init --context
Hook path: node_modules/vibecop/dist/context.js
File exists: YES

610 tests pass, type check clean, lint clean, both builds succeed.

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