[yusiwen]

What changed

• agent: KeptPaths + KeepPath/UnkeepPath/keepsPath; compactRunMessages keeps any round whose
  tool_calls arguments name a kept path verbatim (after the marker, before the tail), summarizes the
  rest, and does nothing at all when every round is kept.
• session: Session.KeptPaths, written by persistSession and restored by adoptSession, so a resume
  keeps the rule.
• TUI: /keep <path>, /keep (list), /unkeep <path>, all refused while a run is active; palette
  entries and /help lines; goldens regenerated.
• Docs: CODEBASE.md (the three functions), roadmap B5 → Delivered — [agent] Give context management turn-level control: pin, unpin, drop #306 (17 delivered, 1 in
  progress, 21 not started, total 40), test count 1285 → 1291.

Why

S4's acceptance was rewritten when S2 landed, because its original sentence — "/drop <path> removes the
tool results that read that path" — assumed a command channel that does not exist: a run's messages are
local to Run, and the TUI refuses commands (which would mutate the slice the run appends to) while it
runs. The rule therefore moves to where it can be made safely: before the run, persisted with the
session.

Kept rounds are re-inserted after the compaction marker rather than in place. That is the same
approximation the pinned-group rule makes for history compression: a summary is already a lossy stand-in,
and keeping the text at all matters more than its exact position in the stand-in.

Verification

• go test ./... -count=1 and -race: 16 packages ok; go vet and gofmt -l clean.
• Mutation checks, both load-bearing paths:
  • the kept-round check disabled → TestCompactRunMessagesKeepsPathsTheUserKept fails with the kept read was summarized away;
  • the persist line dropped → TestKeepPathSurvivesTheSessionFile fails with the session kept [].
• New tests: 3 in agent (list cleaning/dedupe, a kept read surviving while an unkept one is summarized,
  an all-kept run costing no call) and 3 in tui (the three command forms, persist + resume, the run
  guard).
• git log --format='%h %G? %s' → G.

Honest scope

• Matching is a substring of the tool-call arguments: a kept path of a.go also matches data/a.go. For
  a path a person types at the prompt that is a convenience, not a correctness guarantee; a directory
  prefix would be the alternative and was not chosen.
• The list is per session, not per project: the same file kept in one session is not kept in the next
  unless it is still the same session.
• A kept round is kept verbatim: if the read itself is huge, the run keeps paying for it. That is what
  "keep" means, and the tool layer's own truncation is what bounds a single result.

Follow-ups

• [agent] Give context management turn-level control: pin, unpin, drop #306 can close with this: B5 has no open increment left.
• [memory] Align the Hindsight adapter with a real deployment #302 (the Hindsight adapter's contract) still needs a real deployment.

Refs #306, #168