feat: in-flight compaction — compact mid-turn at every step boundary

At the end of every step that produces tool calls (the tool-result
boundary), the orchestrator now checks whether the conversation's context
size has reached the compaction threshold (compact-percent of the model's
context window). If it has, it compacts the history mid-turn — summarizing
old messages and replacing the running history with [summary, ...recent] —
then continues the prompt, so a long-running turn (e.g. left overnight)
does not run out of context.

This mirrors Opencode's approach (check at step boundaries, compact,
continue) and is DISTINCT from the pre-existing post-seal auto-compact,
which runs only AFTER a turn ends and refuses while a conversation is
active. The new path runs DURING the turn (saving the running turn) using
the live step usage rather than persisted metrics (written only at turn end).

Architecture (kernel purity preserved):
- Kernel contract: add an optional, feature-agnostic `onStepBoundary` hook
  to RunTurnInput. The runtime calls it at each tool-result boundary with
  the step's usage + the current messages, and adopts whatever message list
  it returns (undefined/empty = unchanged). Mirrors `drainSteering`.
- Kernel runtime: ~21 lines — call the hook after onStepComplete +
  drainSteering; replace working messages in place when a list is returned.
  No I/O, no feature names.
- Orchestrator: extract a shared `performCompaction` helper (summarize +
  fork + replaceHistory + emit) used by both `compact()` and the new
  in-flight path. Wire `onStepBoundary` in `runTurnDetached` to check the
  threshold via stepUsage vs contextWindow*percent and, on exceed, compact
  and return [summary, ...kernel's recent N] (preserves mid-turn steering
  + the vision-transformed provider view; the store gets the canonical
  recent messages). `compact()` is refactored to use the shared core;
  its active-conversation guard and metrics-based auto-threshold are
  unchanged.

Tests (+13): 8 kernel onStepBoundary tests (replacement adopted by next
step; undefined/empty = unchanged; not called on text-only turns; called
once per tool-call step after drainSteering; receives stepUsage; omitted =
no-op; replacement persists across steps); 5 orchestrator in-flight tests
(triggers above threshold + turn continues; no-op below threshold; no-op
when percent=0; no-op on text-only turns; the manual compact() service
still refuses while active but in-flight runs anyway). Also fixed the
in-memory test store's getCompactPercent/setCompactPercent (were no-ops).

Verification: typecheck clean; 2012 tests pass (was 1999; +13); biome 0
errors (12 pre-existing warnings in untouched files).

0 files changed, 0 insertions, 0 deletions