Agent: Context compaction policy for tool and verification outputs

> **Roadmap:** In-pipeline build/lint fix-up loop; Memory token budget (shipped)
> **Priority:** P2

## Component

Agent (Python runtime)

## Describe the feature

Implement **context compaction** for large tool and verification outputs (build logs, test failures, linter diagnostics) before re-injection into the agent context. Full-fidelity artifacts are offloaded to durable storage with retrievable handles; the active prompt receives structured summaries only.

Addresses arXiv:2605.18747 §3.2.6 (context compaction and state offloading): long-horizon tasks overload context with noise and obscure decision-relevant evidence.

## Use case

- **In-pipeline fix-up loop** (roadmap) needs failure summaries, not 50 KB of raw Maven output, on retry turns.
- **Nudge and long tasks** avoid context explosion from accumulated tool output.
- **Token budget** is spent on actionable signals, not log spam.

## Proposed solution

1. Compaction function in `agent/` (e.g. `context_compaction.py`):
- Input: raw command output + sensor type (`verify_build`, `verify_lint`, shell)
- Output: `{ summary, structured_fields, full_artifact_uri }`
- Structured fields example for test failure: failing test name, key stack frames, suspected files

2. Store full output to session workspace or S3 prefix; emit `TaskEvent` with URI.

3. Blueprint-configurable policy: `compaction: default | aggressive | off` (default: compact on retry paths).

4. Emit OTEL metric: tokens before/after compaction.

### Acceptance criteria

- [ ] Compaction applied when post-hook failures are fed back to agent (when fix-up loop ships) OR on a standalone retry path behind flag for initial delivery
- [ ] Full log preserved at retrievable URI referenced in TaskEvents
- [ ] Unit tests with multi-KB fixture logs; summary preserves failing test name and file paths
- [ ] Documented in `COMPUTE.md` or agent README; token impact noted in `COST_MODEL.md` if measurable

## Other information

- **Complements:** In-pipeline build/lint fix-up loop (roadmap)
- **Paper:** arXiv:2605.18747 §3.2.6
- **Existing:** ~100K hydration budget, 2K memory budget (orchestrator-side)

## Acknowledgements

- [ ] I may be able to implement this feature
- [ ] This might be a breaking change

Start in the agent/ directory and trace how post-hook failures, TaskEvents, blueprint configuration, and session artifacts are handled. Define the standalone retry-path behavior and storage boundary first, then use multi-KB fixture logs to verify that summaries retain failing test names and file paths, full logs have retrievable URIs, and documentation is updated in COMPUTE.md or the agent README.

Written by the indexing model from the issue text.