Issue: Allow custom tool implementations without modifying core source

Problem

Users who want to replace or extend Hermes tool implementations (browser tools,
web tools, terminal backends, etc.) must modify files in tools/. This creates:

• Merge conflicts on every hermes-update.sh --reinstall
• Fork dependency — custom features can't be shared without forking
• Broken plugin model — skills and context engines are already pluggable,
  but tools are not

Multiple users have already built custom browser backends (headed Chrome CDP,
Camofox, alternative automation frameworks) that all require patching
tools/browser_tool.py.

Proposed Solution

Add a ~/.hermes/custom_tools/ directory that Hermes scans at startup.
Any .py file whose module name matches a built-in tool (e.g. browser_tool)
overrides the built-in implementation. New files add new tools.

~/.hermes/custom_tools/
├── browser_tool.py      # Overrides built-in browser tools
├── web_tools.py         # Overrides web search/extract
└── my_tool.py           # Adds a new custom tool

This requires ~30 lines in tools/registry.py and is fully backwards compatible.

Benefits

• Zero core modifications needed for custom tool implementations
• Survives hermes-update.sh --reinstall
• Custom tools are shareable (pip packages, git repos, .py files)
• Consistent with existing skill and context engine plugin patterns
• No breaking changes — directory is optional, defaults to no-op

Use Cases

• Headed Chrome CDP browser automation (replace headless Chromium)
• Alternative search backends (custom search APIs, local indexes)
• Custom terminal backends (specialized execution environments)
• Domain-specific tools (finance, legal, scientific data extraction)

Implementation

PR ready: ironin/tool-override-plugin branch with:

• tools/custom_tools_loader.py — directory scanning + import with override patch
• tools/registry.py — add override=True param to register()
• model_tools.py — call custom tools loader after built-in discovery
• Docs page: website/docs/user-guide/advanced/custom-tools.md

Related Issues

• Feature: Enhanced Extension System with Tool Interception & Lifecycle Events (inspired by Pi) #359 — Enhanced Extension System with Tool Interception (Pi-inspired). Complementary approach: Feature: Enhanced Extension System with Tool Interception & Lifecycle Events (inspired by Pi) #359 proposes runtime interception/hooking of tool calls, while this issue proposes wholesale replacement of tool implementations. Both could coexist — this handles "use a different implementation", Feature: Enhanced Extension System with Tool Interception & Lifecycle Events (inspired by Pi) #359 handles "modify/inspect/block calls to any implementation".
• Feature: MCP Server Management — Discovery, Selective Tool Loading, and hermes mcp CLI #690 — MCP Server Management. Different scope (MCP server lifecycle), but shares the goal of making tools pluggable.
• Feature: Agent Mode System — Persona + Tool Scoping + Behavioral Constraints (inspired by Kilocode) #476 — Agent Mode System / Tool Scoping. About persona-based tool visibility, not custom implementations.
• Define trust semantics for custom CDP/browser-connect endpoints #8084 — Trust semantics for custom CDP/browser-connect endpoints. Relevant security consideration for custom browser tool overrides.