[yusiwen]

What changed

• agent/compression.go: the partition is now a value — compressionPlan (head, middle, pinned, tail,
  tokens, threshold) produced by planCompression, with the reason it cannot be built when it cannot.
  compressHistory executes the plan instead of re-deriving the same boundaries.
• agent/compression_preview.go (new): CompressionPreview() renders the plan — whether automatic
  compression would fire, each kept region's messages (role + first line, capped at 12) and token count,
  and the middle's token count. No provider call.
• tui/update.go: /compress preview prints it; still refused while a run is active (reading a slice the
  run appends to is a data race even though nothing is written).
• Docs: CODEBASE.md (the plan and the preview), roadmap B5 (S1–S3 landed, S4 open), test count
  1277 → 1285.

Why

A compression is destructive and was opaque until it ran. "What would be kept" is a question with a cheap
answer — the partition — and the preview is that answer. Making the compression execute the same plan is
what keeps the answer true: a second implementation of the boundaries would drift, and the drift would
only be visible after a summary had already replaced the turns.

Verification

• go test ./... -count=1 → 16 packages ok; go vet and gofmt -l clean.
• Mutation checks, both load-bearing assertions:
  • a provider call added to the preview → TestCompressionPreviewCallsNoModel fails with the preview called the provider 1 times;
  • pinned and tail swapped in the compression → TestCompressionPreviewMatchesTheCompression fails with
    pinned message 0 differs.
• New tests: the preview calls no model; it names every kept region and reports the middle by size; it
  says why nothing can be compressed (fewer than four turns, disabled, all-pinned middle); the plan and
  the compression agree message for message; the over/under-threshold wording; firstLine clipping; and
  the TUI command, including the run guard.
• git log --format='%h %G? %s' → G.

Honest scope

• The preview describes what a compression would do on the current history. If the history changes
  between the preview and the command (another turn, or a run), the partition changes with it; there is no
  reservation.
• It reports the pinned middle as the pinned region when a plan exists. A middle that is entirely pinned
  has no plan at all — compression would do nothing — so the preview reports the reason instead of empty
  sections.
• No interactive confirmation: /compress preview prints, /compress still compresses without asking.

Follow-ups

• [agent] Give context management turn-level control: pin, unpin, drop #306 S4 (path-targeted unload) is the last increment in B5.
• [memory] Align the Hindsight adapter with a real deployment #302 (Hindsight adapter's contract) still needs a real deployment.

Refs #306, #168