fix(lockfile): stamp the current capa version when pins change

[Minitour]

Summary

Adding a skill after upgrading capa updated capabilities.lock but left generator at the old version (e.g. capa@2.2.0 written by 2.3.0). Closes #310.

What changed

• LockfileBuilder.build() restamps generator with the current version together with generatedAt whenever the pins change.
• A no-op install still leaves the lockfile untouched, so upgrading capa alone doesn't create a lockfile diff in every project.
• Every lockfile write (CLI install, clean, server sync) goes through the builder, so the CLI and the web UI are both covered.

Test plan

• New test: an unchanged rebuild keeps capa@2.2.0; adding a skill stamps the current version.
• bun test src/shared/__tests__/lockfile.test.ts
• bunx tsc --noEmit

Checklist

• Tests added or updated
• bunx tsc --noEmit passes
• Docs updated (if user-facing): not needed

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