feat(compaction): optional compaction.model override + re-enable senpi compaction on SDK-native claude-sdk-oauth lanes

[HeiTuz]

Problem

The claude-sdk-oauth lane keeps a resident Claude Agent SDK session, and the compaction lane policy (lane-policy.ts) stands down senpi compaction entirely on the assumption that the SDK runs its own native compaction. In practice that native compaction never fires for these sessions — senpi's session.log shows threshold/pre_prompt compactions repeatedly rejected with the Claude Agent SDK owns compaction for this session while context grows unbounded (observed past 335k tokens) until overflow, with no recovery path.

Change

Add an optional compaction.model setting ("provider/model", e.g. deepseek/deepseek-chat) that redirects only the compaction summarization call to a different model. When set:

1. Lane policy (lane-policy.ts): the SDK-native stand-down lifts, so senpi auto/threshold compaction runs on the lane again. This is the escape hatch for lanes whose SDK never compacts.
2. Execution (agent-session.ts → _executeCompaction): a new _resolveCompactionModel() resolves the override through the model runtime and uses it for the summarization auth (_getCompactionRequestAuth) + compact() call. Session bookkeeping (lifecycle record, preparation, branch) still tracks the session model, so history/diagnostics are unaffected.
3. Safe fallback: any resolution failure (unset / malformed provider/model / unknown model) falls back to the session model, so compaction can never silently break.

Default behavior is unchanged for every provider — with no override, nothing changes.

Surface

• CompactionSettings.model?: string (settings-manager.ts, compaction/compaction.ts), exposed through getCompactionSettings().
• LaneContext gains an optional getCompactionSettings getter so the lane policy can read the override.

Validation

• New tests: lane-policy stand-down lift with override + continued stand-down when the override resolves to no model; settings model passthrough.
• test/compaction/ suite: 47 files / 339 tests pass.
• Root npm run check (biome + pinned-deps + ts-imports + shrinkwrap + tsc --noEmit): clean.

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Summary by cubic

Adds an optional compaction.model override that redirects only the compaction summarization call to a specified model and re-enables senpi compaction on claude-sdk-oauth lanes when set. Prevents unbounded context growth when SDK-native compaction never fires, with no change to default behavior when unset.

• New Features
  • compaction.model accepts "provider/model" (e.g., deepseek/deepseek-chat); used only for summarization and falls back to the session model if invalid or unresolved.
  • Lane policy: when an override is set on claude-sdk-oauth lanes, senpi auto/threshold compaction runs again instead of standing down.
  • Setting is exposed via getCompactionSettings() and resolved at execution time without affecting session bookkeeping or diagnostics.

^{Written for commit 95f7dfb. Summary will update on new commits.}

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