refactor(tickets): unify ticket plugin behind provider adapters

[BjRo]

Why

Ticket #227 asks for support for another ticket system without duplicating ticket skills or changing their public operation shape. Closes #227.

What changed

Rename the plugin to darrow-tickets and move GitHub Issues behind a contained Python adapter selected before tracker access. Preserve the four public skills and GitHub behavior, allow opaque ticket IDs at the shared boundary, and update specifications, plugin discovery, docs, and the adaptive-delivery eval fixture. This PR does not add a Linear adapter.

Verification

• bun run check:python passed, including 159 ticket package tests with 100% measured line and branch coverage.
• bun run check:python:inventory, bun run check:docs, bun run check:decisions, bun run lint, bun run lint:ts, bun run lint:shell, and bun run typecheck passed.
• The frozen fresh-install probe, five Codex ticket evals, one Claude ticket eval, and the adaptive-delivery ticket preflight eval passed.

Review notes

The plugin identity changes from darrow-tickets-github to darrow-tickets (version 0.5.0). Existing installations need to uninstall the old identity, install the new one, and start a new host session. The provider selection and GitHub-specific reference normalization are the main review points.

Checklist

• I have read and followed CONTRIBUTING.md, including the contribution licensing terms.
• I added or updated the applicable invariant before implementation, or this change does not affect a capability invariant.
• I added or updated colocated evals, or this change does not affect skill behavior.
• I confirmed that each changed plugin remains self-contained, or this change does not affect plugin content.
• I ran bun run check:python, or this change does not affect registered Python packages or their repository quality infrastructure.