Session replay and time travel

Expose a session brain's audit log and versioned KV state as a replay timeline, and let a reader fork a new session from any captured point.

Status

Accepted (standalone companion to ADR 0003 and ADR 0020).

Context

A session brain holds a versioned KV store and an append-only audit log (ADR 0003). Today the human reads the final state and the feed events a session chose to promote, but cannot see how a brain got there, or rewind to inspect an earlier state. Agent adoption and fork (ADR 0020) already create a lineage, yet there is no surface that walks that lineage.

Decision

Add a read-only replay surface over a single session brain:

• The session view gains a "Replay" mode that walks the audit log as a timeline, showing each mutation with its actor, key, and value diff against the previous entry.
• A scrubber (keyboard and pointer) sets the as-of time; the KV panel renders the state at that point, and reading a value opens its version history.
• A "Fork from here" action starts a new named session whose brain is seeded from the captured KV snapshot and recorded as a fork of the original.
• Replays are scoped to readers who may read the session's project, exactly as brain_get and brain_list already are.
• No mutation or rewind of the original brain: replay never writes to it.

Consequences

• Agent mistakes become forensic and reversible; the audit log becomes a first-class UI instead of a debug artifact.
• Adds a per-session snapshot read path. The versioned KV already stores enough to reconstruct state; the scrubber indexes rather than exports.
• Forking produces a new brain with a distinct session id and its own lifecycle (ADR 0020), so GC on prune still works unchanged.
• Keeps the "human is the garbage collector" rule: snapshots remain in the session's brain and are pruned with it.