feat(hermes_state): Gateway session activity tracking so silent stalls are visible

[fangliquanflq]

What does this PR do?

Adds durable mid-turn Gateway session activity tracking so operators can see when a gateway agent turn is still making API/tool/compaction progress, even before new message rows land. This implements suggested feature from #72016 (activity tracking + CLI exposure). It does not add the stall watchdog, compaction timeout, or cumulative stream-retry deadline from that issue.

Motivation

AIAgent already updated an in-memory activity clock on API/tool/compaction work, and the gateway used that for in-process inactivity timeouts. SessionDB last_active for hermes sessions list / hermes status came only from message timestamps (or started_at). During a long or stalled turn with no new messages, CLI surfaces looked idle and gave operators no durable activity signal.

How to see the gap

1. Run hermes gateway with a messaging platform configured.
2. Start a turn that spends a long time in API retries, tools, or compaction without writing assistant messages yet.
3. In another terminal, run hermes sessions list and hermes status.

Desired: listings/status reflect recent agent activity (API/tool/compaction).
Without this feature: Last Active / session status stayed frozen on the last message time; status only showed an active session count.

What this feature adds

• sessions.last_activity_at and SessionDB.touch_session_activity.
• Rate-limited (60s) durable stamps from AIAgent._touch_activity (same cadence posture as the kanban heartbeat bridge).
• Listing last_active as freshest of heartbeat and latest message timestamp (so a lagging heartbeat cannot rank behind newer messages).
• Freshest gateway Last activity line in hermes status.

Related Issue

Related to #72016 (implements suggested item #1: Gateway session activity tracking). Does not close the issue; stall watchdog / compaction timeout / cumulative retry deadline remain open.

Type of Change

• ✨ New feature (non-breaking change that adds functionality)

Changes Made

• hermes_state.py - last_activity_at column, touch helper, freshest-of last_active SQL for list/status/archive paths
• run_agent.py / agent/agent_init.py - rate-limited durable persist from _touch_activity
• hermes_cli/status.py - print freshest gateway last activity
• tests/run_agent/test_session_activity_persist.py, tests/test_hermes_state.py, tests/hermes_cli/test_status.py - coverage for persist cadence, freshest-of ranking, status line

How to Test

1. Manual: during a long gateway turn, confirm hermes sessions list advances Last Active from heartbeats and hermes status shows Last activity.

2. Automated (already run locally, 23 passed):

scripts/run_tests.sh \
  tests/run_agent/test_session_activity_persist.py \
  tests/hermes_cli/test_status.py \
  tests/test_hermes_state.py::TestListSessionsRich \
  -q

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix/feature
• I've run scripts/run_tests.sh on relevant tests and they pass
• I've added tests for my changes
• I've tested on my platform: Windows 11

Documentation & Housekeeping

• I've updated relevant documentation - N/A (docstrings only)
• I've updated cli-config.yaml.example if I added/changed config keys - N/A
• I've updated CONTRIBUTING.md or AGENTS.md if I changed architecture or workflows - N/A
• I've considered cross-platform impact - N/A (SQLite + CLI paths are cross-platform)
• I've updated tool descriptions/schemas if I changed tool behavior - N/A