Pack v2: settle skills discovery and materialization model

[donbox]

Summary

Pack v2 docs define skills/ and agents/<name>/skills/, but the runtime discovery/materialization model is not yet implemented strongly enough to gate.

Why

The directory shape looks stable, but the product contract is still unclear in the places that matter most for conformance: discovery timing, provider projection, precedence, and promotion behavior.

Freeze now

• top-level skills/ as the pack-wide skills directory
• agents/<name>/skills/ as the per-agent skills directory
• skill bundle shape centered on SKILL.md with optional scripts/, references/, and assets/

Decide here

• when pack skills are discovered
• how they are materialized into provider-specific runtime locations
• precedence and collision behavior between pack-wide and agent-local skills
• whether skill promotion is real product behavior or design-only intent

Out of scope for the current suite gate

• pack skills/ discovery as a 0.13.6 runtime conformance requirement
• provider-specific behavior that has not been implemented yet

References

• docs/packv2/doc-agent-v2.md
• docs/packv2/doc-directory-conventions.md
• docs/packv2/skew-analysis.md
• docs/packv2/doc-conformance-matrix.md
• task: migration-validation-suite-first-pass