What does this PR do?

Reframes persisted context-compaction summaries so earlier work is serialized under explicitly historical section headings instead of live-sounding ## Active Task / ## Pending User Asks / ## Remaining Work labels. This reduces the chance that a resumed or long-idle conversation treats stale compaction handoff text as the current instruction.

Related Issue

Refs #42812

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: introduced historical handoff heading constants and updated the compaction prefix, fallback summary body, and LLM summary template to use historical section names.
• tests/agent/test_summary_prefix_semantics.py: added coverage that the prefix references historical headings and no longer names active-sounding sections.
• tests/agent/test_resume_stale_active_task.py: updated resumed-handoff regression fixtures to the new historical task heading.
• tests/agent/test_context_compressor.py: updated fallback-summary assertions to match the historical heading.
• tests/agent/test_context_compressor_temporal_anchoring.py: updated structured-template assertion to match the historical heading.

How to Test

1. Run pytest tests/agent/test_summary_prefix_semantics.py tests/agent/test_resume_stale_active_task.py tests/agent/test_context_compressor.py tests/agent/test_context_compressor_temporal_anchoring.py -q.
2. Confirm the compaction handoff text now uses ## Historical Task Snapshot, ## Historical Pending User Asks, and ## Historical Remaining Work instead of the old active-sounding headings.
3. Optionally reproduce the idle/resume scenario from Stale compaction Active Task hijacks resumed sessions after idle timeout #42812 and verify Hermes answers the latest user message instead of resurfacing the old compacted task.
4. Broad-suite note: local pytest tests/ -q -x --timeout=60 currently aborts during collection on tests/hermes_cli/test_dashboard_auth_401_reauth.py because fastapi is missing in this environment.

What platforms tested on

• macOS on darwin-arm64 (local)

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 darwin-arm64

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

• Targeted tests passed: 110 passed in 8.61s.
• Scoped ruff checks passed on the changed Python files.
• python scripts/check-windows-footguns.py --all passed.
• git diff --check passed.
• Broad suite aborts early on missing fastapi during collection of tests/hermes_cli/test_dashboard_auth_401_reauth.py in this local environment.