fix(dashboard): offload PTY spawn/close off the event loop

[IAvecilla]

What does this PR do?

The in-dashboard chat tab could make the entire dashboard slow and intermittently unreachable, other tabs would hang and the instance would flip to "reconnecting."

Root cause: the /api/pty WebSocket handler (pty_ws) did two blocking operations directly on the single asyncio/uvicorn event loop:

• PtyBridge.spawn(...) — forks + execs a full TUI child process (blocking) on every connect.
• bridge.close() — escalates SIGHUP → SIGTERM → SIGKILL with up to ~1.5 s of blocking waits on every teardown.

Because the dashboard runs a single worker, each of these froze every other request, including the public /api/status liveness probe, for their duration. This was amplified by the PTY auto-reconnect added in #52962: a dropped chat socket now respawns the child, so on a resource-tight machine the connect/teardown cycle (and therefore the event-loop stalls) repeats, dragging the whole dashboard down.

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

• hermes_cli/web_server.py (pty_ws): wrap PtyBridge.spawn(argv, cwd=cwd, env=env) in await asyncio.to_thread(...) so the fork+exec no longer blocks the event loop on connect.
• hermes_cli/web_server.py (pty_ws finally): wrap bridge.close() in await asyncio.to_thread(...) so the SIGHUP→SIGTERM→SIGKILL teardown (up to ~1.5 s) no longer blocks the event loop on disconnect.

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.
• I've run the dashboard/PTY test suites and they pass.
• I've added tests for my changes
• I've tested on my platform: macOS 15 (build/local)

Documentation & Housekeeping

• I've updated relevant documentation (README, docs/, docstrings) — N/A (no public surface change; added explanatory comments inline)
• 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 (Windows, macOS) — asyncio.to_thread is stdlib/cross-platform; no platform-specific behavior changed
• I've updated tool descriptions/schemas if I changed tool behavior — N/A