What does this PR do?

ContextCompressor._previous_summary persists across session boundaries when the same cached AIAgent instance is reused in the gateway or cron. When a new session starts without an explicit /reset, on_session_start() is called but on_session_reset() is not. Since on_session_start() is a no-op on the base ContextEngine, _previous_summary from the old session leaks into the next session's iterative summary update, causing old task context (e.g. "## Active Task: Deploy the web server") to bleed into the new session's compression summary.

This PR overrides on_session_start() on ContextCompressor to clear _previous_summary and _ineffective_compression_count, ensuring a fresh compressor state when a new session begins while preserving the more thorough cleanup in on_session_reset() for explicit /new or /reset.

Related Issue

Fixes #14603

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✨ New feature (non-breaking change that adds functionality)
• 🔒 Security fix
• 📝 Documentation update
• ✅ Tests (adding or improving test coverage)
• ♻️ Refactor (no behavior change)
• 🎯 New skill (bundled or hub)

Changes Made

• agent/context_compressor.py: Override on_session_start() to clear _previous_summary and _ineffective_compression_count.
• tests/agent/test_compressor_session_isolation.py: 3 targeted tests:
  1. on_session_start clears _previous_summary.
  2. on_session_reset still works as before.
  3. Summary state does not leak between sessions.

How to Test

1. Run the targeted suite:

   pytest tests/agent/test_compressor_session_isolation.py -v

   Tested on macOS (Python 3.14).

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(scope):, feat(scope):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes, strongly encouraged for features)
• I've tested on my platform: macOS 15 (Darwin 24.6.0)

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — or N/A
• I've updated cli-config.yaml.example if I added/changed config keys — or N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows — or N/A
• I've considered cross-platform impact (Windows, macOS) per the compatibility guide — or N/A
• I've updated tool descriptions/schemas if I changed tool behavior — or N/A

Screenshots / Logs

N/A — see commit description and PR diff.