feat: add native Wayfinder-style task planning

[pruge]

Summary

Adds a Firstmate-native planning/decomposition layer inspired by Matt Pocock's Wayfinder, to-spec, and to-tickets skills.

Design

• Simple tasks keep the existing direct crew path.
• Clear multi-part work gets spec.md + vertical tickets/.
• Large/uncertain work gets a Wayfinder-style decision map first, then spec and tickets.
• Decision questions are resolved through Firstmate's existing scout/captain-hold mechanisms.
• Implementation is explicitly handed back to Firstmate's existing brief/spawn/supervision lifecycle.
• Wayfinder's implement phase, separate runtime, branch orchestration, and mandatory per-ticket review are intentionally not imported.
• Planning artifacts are private under data/<task-id>/plan/ and expose conclusions/rationale rather than hidden chain-of-thought.

Current scope

This PR adds the internal skill and documentation. The skill's metadata is designed for Firstmate's normal skill discovery. No existing dispatch, brief, spawn, delivery, or merge machinery has been replaced.

Next integration step

Validate the skill in a real Firstmate session with a representative medium/large request, then decide whether an explicit routing sentence in AGENTS.md is necessary after observing the harness's skill-discovery behavior. If needed, add that as a follow-up rather than coupling the first change to a large rewrite of the always-loaded contract.

This is intentionally staged so the existing firstmate lifecycle remains unchanged until the planning behavior is proven in practice.