feat(node-agent): add computer_control desktop automation and capability-based tool discovery

Major integration of desktop automation and structured capability reporting for Hermes Node Gateway.
This enables a unified node execution framework with optional tool support that is
automatically detected and advertised.

Features:
- ComputerController class (Linux/X11) using xdotool and import (ImageMagick)
  - Screenshot (full screen, optional path or base64-return)
  - Mouse: move(x,y), click(button), position()
  - Keyboard: type_text(text), key_press(key like 'Return', 'Ctrl+c')
  - Window: active_window() returns focused window info
- Capability-based registration to gateway
  - Agent sends tool list ["exec", "browser_control", "computer_control"] on connect
  - gateway filters tools based on declared capabilities
  - missing PC deps handled: checks for xdotool/import; browser extension presence
- Universal installer updated:
  - Prompts: enable browser control? enable computer control?
  - Optional per-node sexec permissions quick-edit (allow/deny/ask comma patterns)
  - Writes config.json with enable_browser, enable_computer_control and permissions JSON
- Fixes:
  - agent formerly advertised capability key 'browser'; gateway expects 'browser_control' → aligned
  - installers baked agent compressed with base64 in shell script, no external files

Files:
- node-agent/: new ComputerController class; enhanced hermes_node_agent.py
- ~/.hermes/plugins/hermes-node-gateway/: added COMPUTER_CONTROL_SCHEMA + handlers + routing
- install_hermes_node_universal.sh (merged in repo dist/ location)

Node Agent endpoint type: "computer_control" commands (gateway forwards to node agent)
Gateway: registers tool 'computer_control' with schema and executes cc_result responses handled

Deployed nodes can now:
  - Execute shell commands via sexec (preserves existing allow/ask/deny)
  - Control browsers (if extension installed)
  - Control desktop (if X11 + xdotool + ImageMagick installed)

Hermes Gateway plugin now exposes 4 tools:
  node_list, node_status, node_exec, browser_control, computer_control ✓

Co-authored-by: Lisa <lisa@nexlab.net>
Signed-off-by: Lisa <lisa@nexlab.net>