Persist agent transcripts as reviewable artifacts

[liam-russell]

Problem

Agent work is ephemeral: ACP chat sessions live only as long as the process, and PTY scrollback dies with the terminal. A week later, "why did the agent change this?" is unanswerable, and a PR carries no record of how the change was produced — weak for review, audit, and trust.

Proposal

• Persist transcripts per session (ACP message stream; PTY output buffer) under .sproutgit/ / the workspace DB — machine-local, never committed.
• Browse: a history tab per worktree listing past sessions (agent, duration, cost if known Token/cost usage meters and fan-out budgets #147), readable transcript view; retained after the worktree is deleted, tied to the branch/provenance record.
• PR integration: on create-PR (PR lifecycle depth: draft/ready, per-check results, merge — for humans and agents #138), offer to append an agent-work summary ("N sessions, agents used, key decisions") to the description.
• Search across transcripts (relates Cross-worktree search #101 cross-worktree search).
• Privacy: local-only by default; anything leaving the machine is explicit opt-in.