chore: remove AI session save/resume helpers

Drop the save-session/resume system entirely:

- functions/save_claude_session.fish, save_antigravity_session.fish
  (session-start hook scripts)
- functions/claude-resume.fish, antigravity-resume.fish, code-resume.fish
  (consumers of the .claude_session/.antigravity_session files)

Also updates docs/fish-config.md (removes the three function sections,
drops the session-helper mention from the python3 dependency note) and
docs/fish-config.index (removes the orphaned keyword entries). Generated
html/wiki/man are rebuilt by build-docs.yml on merge.

External hook registrations were removed from ~/.claude/settings.json and
~/.gemini/settings.json (the latter pointed at an already-deleted
save_gemini_session.fish); those files live outside this repo.

docs/fish-config.index

@@ -133,14 +133,11 @@ logging=## 5.11 Pager and Logging
 logs=### logs
 smart-exit=### smart_exit
 ai=## 5.12 AI and Developer Tools
-antigravity=### antigravity-resume
 antigravity-ide=### antigravity-ide
 agy=### agy
 claude-cli=### claude
-claude=### claude-resume
 claude-docs=### claude-docs
 claude-pr=### claude-pr
-code-resume=### code-resume
 superpowers=### superpowers
 media=## 5.13 Media and Utilities
 dng2avif=### dng2avif