feat(tui): add /sessions slash command for browsing and resuming previous sessions

[austinpickett]

What does this PR do?

Adds a /sessions slash command to the Hermes TUI (Terminal User Interface) that allows users to browse and resume previous sessions via an interactive picker. This provides a convenient way to switch between sessions without losing context.

Related Issue

N/A

Type of Change

• ✨ New feature

Changes Made

• ui-tui/src/app/slash/commands/session.ts: Added the /sessions command definition that opens the existing SessionPicker overlay.
• hermes_cli/commands.py: Added CommandDef("sessions", "Browse and resume previous sessions", "Session") to the COMMAND_REGISTRY so the gateway recognizes the command and includes it in autocomplete.

How to Test

1. Check out the branch: git checkout austin-feat-sessions-skills-menu
2. Run the TUI: hermes htui
3. Type /se in the command input — you should see sessions appear in the autocomplete dropdown.
4. Press Enter to open the SessionPicker:
   • Use up/down arrows to navigate
   • Press Enter or number keys (1-9) to resume a session
   • Press 'd' to delete a session (confirmation required)
   • Press Esc or 'q' to cancel
5. Verify that the picker shows a list of previous sessions (up to 200) and that selecting one resumes it correctly.

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 (no regressions)
• I've tested on my platform: Ubuntu 24.04 (WSL2)

Documentation & Housekeeping

• I've updated relevant documentation (this PR description explains the feature)
• I've considered cross-platform impact (changes are UI-related, should work on all platforms)
• I've updated tool descriptions/schemas if I changed tool behavior (N/A — no tool behavior change)

For New Skills

N/A

Screenshots / Logs