docs: Write Usage guides (× 7)

[shouze]

Context

Parent epic: #24
Depends on: #25 (VitePress initialized)

Pages to create in docs/usage/

search-syntax.md

• GitHub code search qualifiers: org:, repo:, path:, language:, filename:, extension:
• Searching 1 to N specific repos instead of the whole org: use repo:org/name qualifiers directly in the query string
• Combining multiple qualifiers
• Link to docs/reference/github-api-limits.md for hard constraints

interactive-mode.md

• TUI layout: repo list, fold/unfold, stats bar, help overlay
• Full keyboard shortcuts table (reference docs/reference/keyboard-shortcuts.md)
• Filter bar: press f, type a path substring, confirm/cancel
• Select all / select none (context-aware: repo row vs extract row)
• Confirmation and replay command output

non-interactive-mode.md

• --no-interactive flag and CI=true env var (three equivalent ways)
• Piping output to a file or another command
• Using --exclude-repositories and --exclude-extracts as a replay mechanism in CI

output-formats.md

• --format markdown (default) vs --format json
• --output-type repo-and-matches (default) vs --output-type repo-only
• JSON schema with a typed example
• Markdown output example with <details> replay block

filtering.md

• --exclude-repositories: short form (repoA,repoB) and long form (org/repoA)
• --exclude-extracts: format repoA:src/foo.ts:0,repoB:lib/bar.ts:1
• --include-archived: why archived repos are excluded by default
• TUI file-path filter bar (link to interactive-mode.md)

team-grouping.md

• --group-by-team-prefix <prefixes> (e.g. squad-,chapter-)
• Required token scope: read:org
• How the team → repo mapping is fetched and cached
• Cache location per OS and GITHUB_CODE_SEARCH_CACHE_DIR override
• Grouping algorithm: single-team, multi-team combinations, other fallback
• Section headers in TUI and in markdown output

upgrade.md

• github-code-search upgrade subcommand
• What it does: fetches latest GitHub release, replaces binary in-place
• Manual upgrade: re-run the curl install script

Acceptance criteria

• All 7 pages exist and render without errors
• Each page has at least one concrete, copy-pasteable CLI example
• Cross-links between pages resolve correctly (no 404)
• search-syntax.md links to docs/reference/github-api-limits.md