Add Superpowers plugin integration

[baljanak]

Summary

• Auto-seed methodology decisions: On SessionStart, detect Superpowers via installed_plugins.json and seed 8 decision files documenting the methodology (TDD, design-first, systematic debugging, subagent-driven development, etc.). Uses a general SeedRegistry extensible to other plugins. Idempotent via .seeded.json manifest — deleted decisions stay deleted.

• Extract decisions from specs/plans: Extend plan-nudge to watch docs/superpowers/specs/ and docs/superpowers/plans/ for decision language. When the agent writes a superpowers spec with trade-offs or approach choices, candidates are extracted and surfaced as a nudge on the first implementation edit. Adds _APPROACH_RE for superpowers-style "Approach 1: ..." sections. dispatch.sh carve-out ensures the /docs/ skip pattern doesn't block these paths.

Test plan

• make check passes (614 tests, ruff, mypy, shellcheck)
• Seed validation: all 8 superpowers seeds pass Decision.validate()
• Detection: registry detects superpowers from various marketplace keys
• Idempotency: second seed call creates 0 files; deleted files not re-created
• Spec extraction: superpowers spec with approaches → candidates extracted
• Nudge message: references superpowers source name and type (spec vs plan)
• Isolation: autouse fixture prevents seed leakage across unrelated tests

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