feat(compact): reactive compaction with direct input support

[crstrn13]

Summary

Completes the remaining work items from #30:

• Use previous_usage for token estimation — should_compact() checks the stored usage.total_tokens from the rehydrated response before falling back to local tiktoken counting
• Configurable summary prefix — summary_prefix config option (default: [Previous conversation summary]) carried through to compaction items and both translation paths
• Hide compaction items from input_items API — normalize_input_items() filters out {"type": "compaction"} items so clients never see internal state
• Persist compaction as a stored response — after summarization, the compacted response is written back to the response store with the compaction item in its input
• Explicit compact endpoint — POST /v1/responses/{id}/compact triggers compaction on a previously stored response

Test plan

• cargo test -p praxis-ai-apis — new unit tests for all 5 items
• cargo test -p praxis-ai-filters — no regressions in filter tests
• make lint passes
• Manual test with example config against a real inference backend

[praxis-bot]

Re-review (Aug 24 commits)

The two new commits fix the summarization scope (exclude current input from summarization text in the rehydrated path), add is_compactable unit tests, add an explicit compact endpoint integration test, fix the stale docstrings on CompactFilter and the docs page, and add store initialization for the compact endpoint.

Previous findings addressed:

• Stale docstrings: fixed (struct doc and docs page updated)
• Missing is_compactable / ensure_compactable_state tests: fixed (extracted is_compactable and added 5 tests)

One new finding below.

Severity	Count
Medium	1

[leseb]

@crstrn13 please rebase