[Feature] Add npm-first update notices and safe `update` command

[mbe24]

Design

Add a lightweight, non-disruptive update system:

1. startup notice from cached update metadata,
2. explicit 99problems update status command,
3. optional 99problems update --apply for npm installs.

Use npm registry metadata as version source. Keep checks configurable via .99problems under [system] and ensure network failures never break normal command flow.

Scope

• Add update subcommand and --apply behavior.
• Add install-channel detection via NINETY_NINE_PROBLEMS_INSTALL_CHANNEL.
• Add cache-backed notice metadata under OS cache dir.
• Add [system] settings: update_enabled, update_interval_days.
• Add env override: NINETY_NINE_PROBLEMS_DISABLE_UPDATE_CHECK=1.
• Trigger background refresh during get only; startup remains cache-only.
• Update README and regenerate man pages.

Boundary

• No auto-apply during normal command runs.
• No package-manager abstraction beyond npm for apply in v1.
• No fallback auto-apply via cargo or release binaries.
• No telemetry upload or analytics for update checks.

Acceptance Criteria

• Startup can print cached update notice without blocking on network.
• get performs best-effort background refresh when cache is stale.
• 99problems update performs check and prints status/guidance only.
• 99problems update --apply:
  • succeeds only for npm channel,
  • fails clearly for non-npm/unknown channel,
  • fails clearly when npm is unavailable on PATH.
• Cached update notice is cleared only when:
  • a check confirms current version is up to date, or
  • update --apply succeeds.
• Offline/timeouts never fail user command flow.
• .99problems [system] settings and env override behave as documented.

Context

This improves operability for both direct users and agent-driven usage, while keeping startup latency low.

Manual update avenues remain available:

• npm global reinstall (npm install -g @mbe24/99problems)
• cargo reinstall (cargo install problems99 --force)
• release binary download path (when published)

References:

• npm package metadata API: https://github.com/npm/registry/blob/master/docs/REGISTRY-API.md
• npm dist-tags behavior: https://docs.npmjs.com/cli/v10/commands/npm-dist-tag
• dirs crate (cache/config paths): https://docs.rs/dirs/latest/dirs/