feat: add :checkhealth claudecode health check

[ThomasK33]

Summary

Adds :checkhealth claudecode support so users can diagnose their integration with Neovim's built-in health framework ("doctor").

New module lua/claudecode/health.lua checks, in order:

Check	Severity on failure
Neovim ≥ 0.8.0	error
setup() was called (+ plugin version)	error
Claude CLI resolves (terminal_cmd or claude on PATH), + CLI version	error
Terminal provider sanity (snacks installed, external cmd valid, auto fallback)	error / info
WebSocket server running + port	warn
Lock file present in ~/.claude/ide/ (or $CLAUDE_CONFIG_DIR/ide/)	error
Claude client connected (handshake-aware via is_claude_connected())	info

Following checkhealth conventions the check is synchronous and side-effect free: it does not launch a Claude instance, but verifies every prerequisite for launching and reports live connection state, with actionable advice on each failure. Includes a vim.health/require("health") compat shim for Neovim < 0.10.

No changes to existing modules — all required state was already exposed.

Testing

• 8 new unit tests in tests/unit/health_spec.lua; full suite green (648/648)
• luacheck + StyLua clean
• Dogfooded in headless Neovim against a live server:

claudecode.nvim ~
- ✅ OK Neovim >= 0.8.0
- ✅ OK claudecode.nvim 0.2.0 is set up
- ✅ OK Claude CLI found: claude (/tmp/coder-script-data/bin/claude)
- CLI version: 2.1.173 (Claude Code)
- ✅ OK Terminal provider 'auto': snacks.nvim not installed, will fall back to native terminal
- ✅ OK WebSocket server running on port 17480
- ✅ OK Lock file present: /home/coder/.claude/ide/17480.lock
- No Claude Code client connected yet (launch one with :ClaudeCode)

Also verified failure paths: setup() not called (error + advice), missing CLI (error + advice), server not running (warning + advice).

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Generated with mux • Model: anthropic:claude-fable-5 • Thinking: high