docs(index): expand help-index coverage and fix stale targets
Add tool-name synonyms whose names appear in no heading (so the
fallback substring scan can't reach them): eza/exa/lsd, trash, btop,
prettyping, duf/dust, kitten, bat. Add integration and topic keywords
(zoxide, direnv, venv/virtualenv, wakatime, tailscale, done/notify,
pager-hierarchy, shell-aliases, kitty-logging/watcher, agy,
antigravity-ide, html/browser, wiki).
Fix stale entries pointing at renamed/removed headings:
- antigravity -> ### antigravity-resume (### antigravity was removed)
- man-page/manpage -> ## As a man page (dropped '(if compiled)')
- disambiguate duplicate 'secrets' key: secrets.fish is now secrets-file
rootiest
1aac43af62
