Add an "update available" notice (install-method aware)

[alexkroman]

Summary

Add an unobtrusive "update available" notice in the npm update-notifier / Vercel style. When a newer release exists, the next human, interactive run prints a small bordered box on stderr after the command finishes, with the upgrade command matching how the CLI was installed.

  ╭────────────────────────────────────────────────╮
  │                                                  │
  │   Update available  0.1.0 → 0.2.0                │
  │   Run  brew upgrade assembly  to update          │
  │                                                  │
  ╰────────────────────────────────────────────────╯

• Never blocks. The notice always renders from a config.toml cache (zero latency). The cache is refreshed by a detached assembly _update-check process (≤ once/day), reusing the exact detached-spawn pattern from telemetry.py.
• Version source = GitHub Releases API (/releases/latest → tag_name) — the PyPI name is squatted, and releases ship as git tags/bottles. releases/latest excludes pre-releases.
• Install-method aware: detects brew / pipx / uv-tool from the running interpreter's path and shows brew upgrade assembly / pipx upgrade assembly / uv tool upgrade assembly (or a docs hint when unknown).
• Suppressed under --json, non-TTY (piped/redirected), CI, AAI_NO_UPDATE_CHECK, or when not actually behind — so pipelines and agents stay clean.
• Best-effort: all network/parse/config failures are swallowed; the notice can never delay or break a command.
• Declares packaging as a direct dependency (already resolved transitively) for robust version compare.

Also includes scripts/bump_minor.sh (a release helper that bumps the minor version, complementing cut_release.sh).

Test Plan

• ./scripts/check.sh → All checks passed.
• aai_cli/update_check.py at 100% coverage; diff-cover 100% on changed lines
• Mutation gate: 26 mutants on changed lines, no survivors (behavioral asserts for the interval value, timeout/follow-redirects passed to the request, empty/missing-tag handling, and every suppression rule; cosmetic Panel styling marked # pragma: no mutate)
• End-to-end: a real command shows the box on a human/TTY run and is silent under --json
• Hidden _update-check command runs and is absent from --help

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