Add local tool integration commands

[breznknecht]

TL;DR

Adds short-form CLI commands for managing HeadsDown integrations in local agent tools.

Changes

• Adds hd install <tool> for claude, pi, and codex.
• Adds hd install --all, --dry-run, and --yes behavior for detected supported tools.
• Changes hd update to refresh installed HeadsDown integrations, with hd update <tool>, hd update --all, and hd update --cli for the CLI binary self-update path.
• Adds hd remove <tool> for removing only HeadsDown-owned integration artifacts.
• Extends hd doctor with integration health checks and hd doctor <tool> / hd doctor --all.
• Hardens doctor output so it reports derived facts only instead of local paths, token fragments, or profile details.
• Updates README examples to lead with the short command shape.

Safety notes

• Install means installing HeadsDown integration artifacts, not installing Claude Code, Pi, or Codex themselves.
• User-owned config is preserved. The CLI only overwrites/removes artifacts with HeadsDown-managed markers or known HeadsDown package entries.
• JSON bulk operations do not print confirmation prompts to stdout.

Validation

• cargo fmt --check
• cargo test
• cargo clippy --all-targets -- -D warnings
• git diff --check