feat: optional Linear-first mode for Conductor tracks

[swainjo]

Summary

• Adds optional Linear-first mode: when a project has conductor/linear.md, tracks use a Linear issue as the spec (pointer metadata.json + plan.md); file-based spec.md / tracks.md remains the default when that file is absent.
• Ships plugin skills (linear-issues, linear-review, linear-handoff, linear-label-review), setup opt-in that writes linear.md and copies CLI fallback scripts, and Linear-aware branches in the engine lifecycle skills.
• Includes GraphQL CLI assets with tests (24 passing) and README docs for MCP → CLI → ask-the-user.

Test plan

• uv run --with pytest pytest skills/conductor-setup/assets/linear/tests/
• Confirm projects without conductor/linear.md still follow file-based new-track / implement / status
• With a temp conductor/linear.md, confirm new-track offers create-or-link and writes pointer track files only
• Spot-check engine skills for Linear-first sections and that Done is never auto-set
• Reload Conductor plugin / new agent session so linear-* skills are discoverable

Made with Cursor

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[swainjo]

Closing: this change is intended for the swainjo/conductor fork only, not upstream.