feat(packs): workflow graph + per-project phase gate bindings

[simonperegrinvs]

Summary

• Visualize a selected guidance pack's workflow (phases + gates) in the Desktop Packs tab (Vue Flow).
• Bind per-phase validation scripts via native file picker; persist per project in .forge/phase-gates.json.
• Seed default bindings from the guidance pack (manifest.json default_phase_gate_bindings) and ship policy-generated default scripts under scripts/phase-gates/*.sh.

Notable API/Behavior

• Desktop backend:
  • GET /api/dialog/select-file?projectRoot=... (filters to .sh)
  • GET /api/phase-gates?projectRoot=...
  • PUT /api/phase-gates (stores project-relative paths; rejects scripts outside project root)
• Pack content now includes defaultPhaseGateBindings.
• Guidance install seeds .forge/phase-gates.json if missing and never overwrites.

Testing

• bun run verify
• cargo test --manifest-path apps/desktop/src-tauri/Cargo.toml