Add run artifact reopen, resume, and retry UI

[WalksWithASwagger]

Context

Run artifacts now exist on disk, but users need a workspace to reopen them, retry failed handoffs, resume partial runs, and compare outputs.

Source: docs/WHISPERFORGE-MASTER-PLAN-2026-05-18.md (wf-run-workspace, Phase 8).

Linear: BC-229.

Acceptance Criteria

• Add a run browser that reads local manifests and shows status, created time, input type, recipe/settings, exports, and errors.
• Allow reopening a run into the output/review surface from its manifest.
• Allow retrying safe downstream handoffs such as markdown export or Notion save without re-running transcription/content stages.
• Represent partial runs clearly and avoid corrupting history records.
• Add docs for recovery behavior and known limits.

Tests/Evals

• Unit tests for manifest loading, partial-run status, and retry-safe operations.
• Rendered UI smoke if the run browser changes visible UI.

Verification

• make test
• venv/bin/python tests/ui_smoke.py
• git diff --check

Agent Instructions

Build on existing .cache/runs manifests and history upserts. Treat destructive cleanup as out of scope.

Out of Scope

Do not implement arbitrary stage replay until reopen/retry is reliable.