fix(check-completion): pass CI gate when plans directory is missing

[vipentti]

Summary

• Fix planlet check-completion --base <git-ref> to treat missing plans/ directory as no plans found (ok: true, exit 0) instead of plans_not_initialized error. src/core/check-completion.ts now catches plans_not_initialized from validatePlanlets and returns empty report after listDiffPaths succeeds, preserving existing behavior when directory exists.
• Add two integration tests covering missing plans/ with and without diff.
• Add CHANGELOG.md Unreleased Fixed entry.

CLI behavior

• Commands/options: check-completion --base <git-ref> unchanged options.
• Output or errors: before plans_not_initialized with exit 1 when no plans/; after ok:true, touched:[], completed:[], violations:[] with exit 0 and empty stderr. Violations still fail with exit 4 when directory exists.
• Compatibility: strictly less failing; CI gate passes in repos without planning until they start planning.

Before:

error{code,message,path}:
  plans_not_initialized,"Repository does not contain a plans directory",/.../plans
Error: Process completed with exit code 1.

After (no plans/):

ok: true
base: base
touched: []
completed: []
violations: []
# exit 0

Verification

• npm run format:check
• npm run lint
• npm run knip
• npm run type-check
• npm run build
• npm test (252 pass, 0 fail after clearing npm_config_allow_scripts env pollution from ~/.npmrc; packaging test passes when env not inherited)
• git diff --check
• Targeted CLI verification: exercised dist/planlet.mjs check-completion --base base on temp repos without plans/ (exit 0, ok true) and with ready plan (exit 4, violation)

Contributor checklist

• Tests cover new or changed behavior
• User-facing documentation and CLI help are updated, or no update is needed (no help change)
• CHANGELOG.md: [Unreleased] entry added
• Every planlet task this PR completes is checked ... or the PR completes no planlet task
• Canonical skill changes have been regenerated ... or no skill changed
• The change preserves supported Node.js and operating-system compatibility