docs(spec): add prioritized improvement roadmap

[talgolan]

Summary

Adds meta/superpowers/specs/2026-06-15-improvement-roadmap.md, a prioritized backlog of 14 improvements across capability, performance, scalability, and security. Derived from a full read of the v0.8.0 implementation plus the 2026-05-21 usage-feedback doc, excluding items that already shipped (3x flaky-test retry, duplicate-number guard, the four end-session heuristics, the outstanding-items overlay, the "Last entry" auto-bump, the session-start status line).

Each item carries problem / change / effort / risk / acceptance criteria. Ordered by leverage and grouped into tiers:

• P0 quick wins — learnings-surface early-exit, version-tag validation before echo, ReDoS bound on the trigger loop.
• P1 features — SessionEnd close-out nudge, secret-write gate, /session-continuity:doctor, auto-generated Symptoms index.
• P2 structural — primer volatile/stable split, archive closed outstanding items, conflict-resistant LEARNINGS IDs + slug cross-refs.
• P3 maintainability — remove legacy docs/ paths at v1.0, document the repo-trust model, trigger authoring aids, generalize the version-sync guard.

Also records the inherited constraints (no new deps, stage-don't-commit, small surface), a suggested release grouping (v0.8.x → v1.0), and three open questions that need a maintainer decision.

This PR is the spec only — no behavior changes. It's a planning artifact under meta/, not shipped to users.

Test plan

• Render the doc on GitHub and confirm headings, code fences, and tiers display correctly.
• Sanity-check that no item duplicates something already shipped in v0.5.1–v0.8.0.
• Confirm file lives under meta/superpowers/specs/ per the repo's CLAUDE.md convention (not docs/).

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