thv llm: tool adapter registry and per-tool implementations

[yrobla]

Context

Part of #5016 (RFC: stacklok/toolhive-rfcs#70). Depends on #5028 (token lifecycle) and #5029 (proxy).

Scope

Registry interface:

• Detect() bool, Apply(config) error, Revert() error interface for tool adapters
• Auto-discovery registry — adding a new tool requires only implementing the interface and registering; no changes to setup/teardown orchestration

Token helper (direct) mode tools:

• Claude Code — patches apiKeyHelper and base URL in ~/.claude/settings.json
• Gemini CLI — patches token command and base URL in Gemini CLI's config

Proxy mode tools:

• Cursor — patches base URL and placeholder API key in Cursor's config
• VS Code (GitHub Copilot) — patches base URL and placeholder API key in VS Code's settings
• Apple Xcode — patches base URL and placeholder API key in Xcode's config

Acceptance Criteria

• AT: Claude Code is configured as a token helper tool — setup patches apiKeyHelper and base URL in ~/.claude/settings.json
• AT: Gemini CLI is configured as a token helper tool — setup patches token command and base URL in Gemini CLI's config
• AT: Cursor is configured as a proxy-mode tool — setup patches base URL and placeholder API key in Cursor's config
• AT: VS Code (GitHub Copilot) is configured as a proxy-mode tool — setup patches base URL and placeholder API key in VS Code's settings
• AT: Apple Xcode is configured as a proxy-mode tool — setup patches base URL and placeholder API key in Xcode's config
• Adding a new tool requires only implementing Detect/Apply/Revert and registering — no changes to orchestration, proxy, or token source

Dependencies

• thv llm: package skeleton, cobra commands, and config management #5027 (foundation: config types, package structure)
• thv llm: OIDC token lifecycle and thv llm token command #5028 (token source, for token helper tools)
• thv llm: localhost reverse proxy for proxy-mode tools #5029 (proxy, for proxy-mode tools)

References

• Parent issue: thv llm subcommand for LLM gateway authentication #5016
• RFC: RFC-0070: thv llm subcommand for LLM gateway authentication toolhive-rfcs#70