Move Taskboard and Usage Tracker to Git releases

[MateoCerquetella]

What changed

• Retires npm distribution for both Taskboard and Usage Tracker.
• Marks both plugin workspaces private and removes publishConfig, package
  allowlists, prepack hooks, and the tracked npm publish credential config.
• Replaces active npm badges, links, and install commands with immutable Git
  semver sources and BB Community shorthand.
• Advances Taskboard to 0.3.1 and integrates the Git-only distribution
  capability while preserving existing release tags and historical evidence.

Verification

• Root npm run check passes.
• Taskboard: SDK check, TypeScript, 89/89 tests, production build, and build
  metadata verification pass at 0.3.1.
• Usage Tracker: SDK check, TypeScript, 13/13 tests, and production build pass
  at 0.1.2.
• Both npm publish --dry-run --ignore-scripts workspace checks refuse
  publication because each workspace is private.
• npm registry tombstones confirm all prior versions of both package names are
  unpublished.
• Empirical verification, security review, independent code review, capability
  integration, and historical receipt-artifact audit pass.

Release plan

After merge, create new immutable annotated tags taskboard/v0.3.1 and
usage-tracker/v0.1.2 at commit
ce9a73401f7bd448f347f6dc6b9d7ae81565df6e; never move existing tags.