fix: align session handling with upstream gastown

[web3dev1337]

Summary

This PR fixes the main remaining drift between gastown-gui and upstream steveyegge/gastown.

Upstream gastown now uses tmux session names derived from rig prefixes in mayor/rigs.json instead of hardcoded legacy gt-* names. The GUI still had multiple server-side assumptions built around the old naming, which broke running-state detection and tmux-backed actions against current upstream towns.

Changes:

• add registry-aware session-name parsing and generation in server/domain/session/SessionNames.js
• update server.js tmux-backed routes to resolve current upstream session names
• update StatusService running-state detection to parse current upstream tmux sessions
• require rig prefixes when deriving agent session names
• document the upstream sync audit in docs/ORCHESTRATOR-SYNC-AUDIT-2026-03.md
• add unit coverage for registry-based session naming and status parsing

Verification

• npm test
  • 37 test files passed
  • 321 tests passed

Upstream References

Checked against:

• steveyegge/gastown 3b4460b
• steveyegge/beads 27b24a2