Feat: cache GitHub team list (24 h) with --no-cache option

[shouze]

Motivation

fetchRepoTeams can fire dozens of GitHub API requests on every run (one page per team × N teams). The team list is quasi-static (changes rarely). Caching it for 24 h eliminates those calls on repeated runs.

Changes planned

• Create src/cache.ts with pure helpers:
  • getCacheDir(): string — OS-aware cache directory (macOS: ~/Library/Caches/github-code-search, Linux: $XDG_CACHE_HOME/github-code-search or ~/.cache/github-code-search)
  • getCacheKey(org, prefixes): string — deterministic file name (org + sorted prefixes)
  • readCache<T>(key): T | null — returns null if file missing, unreadable, or older than 24 h (based on mtime)
  • writeCache<T>(key, data): void — creates directory if absent, writes JSON
  • CACHE_TTL_MS = 24 * 60 * 60 * 1000 exported constant
• Create src/cache.test.ts (TTL expiry, deterministic key, missing directory)
• Update fetchRepoTeams(org, token, prefixes, useCache?) in src/api.ts to read/write cache
• Add --no-cache flag to addSearchOptions in github-code-search.ts; print a dim message on stderr when cached data is used
• Update README.md: add Cache section with cache location per OS and purge commands

Cache location

OS	Path
macOS	~/Library/Caches/github-code-search/
Linux	$XDG_CACHE_HOME/github-code-search/ or ~/.cache/github-code-search/

Purge commands

# macOS
rm -rf ~/Library/Caches/github-code-search

# Linux
rm -rf ${XDG_CACHE_HOME:-~/.cache}/github-code-search

Acceptance criteria

• bun test — all tests green
• bun run lint — zero oxlint errors
• bun run format:check — zero diff
• bun run knip — no unused exports
• bun run build.ts — binary compiles

Dependencies

Depends on #17 (src/api-utils.ts must be merged first).