feat: add context compaction analysis

[Siddhant-K-code]

Summary

• add agent-strace compaction to detect provider-reported context drops and estimate discarded-context cost
• diff retained versus likely-dropped goals, constraints, decisions, reads, and modified files per conversation sidechain
• compare pre/post-compaction lint and tool behavior, including re-reads, loops, exploration, and context saturation
• add post-compaction-regression to agent-strace lint
• add live pre-compaction Markdown checkpoints with per-stream, O(1) watcher state and retention cleanup
• preserve Claude JSONL usage, compaction summaries, and stable sidechain identity during import
• document analysis, checkpoint privacy, flags, and interpretation; bump the package to 0.89.0

Closes #217

Verification

• python -m unittest tests.test_compaction tests.test_jsonl_import -v (36 passed)
• Ona test task (1,765 Python tests passed; 3 VS Code tests passed)
• python -m py_compile src/agent_trace/compaction.py src/agent_trace/jsonl_import.py
• git diff --check

[Siddhant-K-code]

Independent review completed against the integrated diff.

Review findings fixed before opening this PR:

• preserved real Claude JSONL usage and compaction summaries instead of relying on synthetic request data
• isolated detection, diffs, behavior windows, and watcher state by conversation sidechain
• excluded estimated payload sizes from compaction reporting, pricing, and startup checkpoints
• replaced the live watcher rescan with O(1) per-stream request/response state, including interleaved responses
• included modified-file context and the full registered lint delta without recursive lint execution
• removed boundary-summary overwrite and checkpoint-retention edge cases

No blocking findings remain. The reviewed revision passes 1,765 Python tests and 3 VS Code extension tests.