feat(cli): add /doctor command for environment and dependency diagnostics

[nilaymallikk]

Summary

Closes #1119.

Adds a /doctor slash command (with aliases /health and /check) to the CLI that runs fast local diagnostics to verify that required tools and environment prerequisites are healthy:

• Runtime & OS: Reports Bun/Node version and platform architecture.
• Git: Validates git binary and whether the current directory is an active Git repo (with branch name).
• Ripgrep (rg): Resolves the bundled ripgrep executable from @codebuff/sdk and verifies it executes properly (with fallback to system rg).
• Tmux: Verifies tmux availability for detached sessions.
• Network Connectivity: Fast reachability check to the configured API endpoint.

Scope

All changes are strictly scoped within cli/:

• cli/src/commands/doctor.ts (new)
• cli/src/commands/command-registry.ts
• cli/src/data/slash-commands.ts
• cli/src/commands/__tests__/doctor.test.ts (new unit tests)

Verification

• Passed unit tests: bun test cli/src/commands/__tests__/doctor.test.ts
• Passed full CI build and packaging: bun run ci
• Passed binary smoke test: bun cli/scripts/smoke-binary.ts cli/bin/freebuff