[yusiwen]

What changed

• types.Message.Pinned — a pin rides on the message, so the compressor can see it.
• agent/context.go: PinLast(n) / UnpinLast(n) / DropLast(n) / PinnedTurns() over turns (a
  user message plus everything up to the next — the grouping compressHistory already works in).
  DropLast copies the retained prefix rather than truncating in place, because history slices are
  shared with goroutines reading them.
• agent/compression.go: a pinned group is never fed to the summarizer and is re-inserted verbatim after
  the summary; a middle that is entirely pinned returns the history unchanged with no provider call.
• TUI: /pin [n], /unpin [n], /drop [n], palette entries, /help lines, and the same
  "a run is in progress" refusal /compress has.
• Docs: CODEBASE.md (compression step 5, new context.go section), roadmap B5 → In progress — [agent] Give context management turn-level control: pin, unpin, drop #306
  (S1 landed, S2/S3 open), palette goldens, test count 1237 → 1249.

Why

The compressor had two settings — summarize the middle, or raise the threshold until nothing is
compressed — and no per-turn control. /drop is context shaping, not deletion: the session file is what
--resume reads, so the command says it leaves the record alone. Pins are the other half: a turn a user
marked is kept verbatim, and the summarizer never sees it.

Verification

• go test ./... -count=1 → 16 packages ok; go vet and gofmt -l clean.
• Mutation check: pin handling disabled (if false around groupPinned) →
  TestCompressionKeepsPinnedGroupsVerbatim fails with the pinned turn was fed to the summarizer;
  restored, it passes.
• The mutation check found a real hole in the test first: the original version pinned turn 3 of 6, which
  sits in the head — the region kept regardless — so it passed against the mutation. It now uses
  eight turns and pins a middle turn, so the assertion is about the path it claims to test.
• New tests: 5 in agent/context_test.go (pin/unpin/drop counts and effects, the pinned group kept
  verbatim and absent from the summarizer, the fully pinned middle costing nothing) and 7 in
  tui/context_command_test.go (each command, counts, empty history, the run guard, the palette).
• git log --format='%h %G? %s' → G.

Honest scope

• A pin lasts for the session: persistSession rebuilds the stored messages from m.messages, so the
  flag has to travel through the UI transcript to survive a resume. That is S2 of [agent] Give context management turn-level control: pin, unpin, drop #306, together with
  the run-level compaction D7 depends on.
• compressHistory still runs only at the start of a run. A long round grows until the provider
  refuses — the gap S2 exists for.
• No file-level unload: file contents are tool results inside a run, not history turns; S2 is where that
  becomes meaningful.
• No compression preview (S3).

Follow-ups

• [agent] Give context management turn-level control: pin, unpin, drop #306 S2 (run-level compaction + pin persistence), S3 (preview).
• B4 remains the open entry in section B; [memory] Align the Hindsight adapter with a real deployment #302 owns the Hindsight adapter's contract.

Refs #306, #168