Plan hygiene: move shipped plans to `docs/plans/done/`

[lbliii]

Saga: #329 · Epic: #332

Several plans are fully shipped but still marked "proposed" or "active": Showcase v2, Premium Polish, AI Chat saga.

Goal

Reduce planning noise per roadmap workstream 2: active plans describe real remaining work, not shipped history.

Tasks

• Move to docs/plans/done/: PLAN-showcase-v2-saga.md, PLAN-premium-polish-saga.md, PLAN-ai-chat-agent-ui-saga.md (if complete).
• Rewrite partially shipped plans as residual backlogs (add "Residual Work" section).
• Update docs/INDEX.md in-flight work section.
• Update docs/strategy/roadmap-pre-1.0.md Active Plan Mapping if needed.

Acceptance

• docs/INDEX.md lists only genuinely active plans under in-flight work.
• No shipped plan contains unqualified Status: active without residual section.

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Plan: docs/strategy/roadmap-pre-1.0.md workstream 2.