feat(deps): catalog python3 as a recommended dependency

python3 is used unguarded by the AI session helpers and (guarded) by the
paru/yay log cleaner, but was absent from the dependency catalog and docs.
uv does not provide python3 on PATH, and Arch's base omits it, so it is
tracked separately rather than assumed via uv.

- Add python3 (recommended tier, pm: python) to _fish_deps_catalog.fish
- Guard python3 calls in save_claude_session/save_antigravity_session with
  `type -q python3`, emitting valid empty JSON when absent (Convention §6)
- Document python3 in docs/fish-config.md (§5.6 tiers + catalog entry)

docs/fish-config.md

@@ -855,7 +855,8 @@ Add -i (interactive confirmation) to destructive commands:
                     direnv, paru/yay
       Integrations  wakatime, tailscale
       Recommended   eza, lsd, bat, btop, dust, duf, prettyping, ov,
-                    ripgrep, lazygit, lazydocker, trash, kitty, wezterm
+                    ripgrep, lazygit, lazydocker, trash, kitty, wezterm,
+                    python3
 
     fish-deps
     fish-deps install
@@ -1380,6 +1381,11 @@ fish-deps manages these tools. Run `fish-deps` to check status, or
     trash       Safe delete (trash-cli)
     kitty       GPU-accelerated terminal (primary)
     wezterm     GPU-accelerated terminal (alternative)
+    python3     Standalone interpreter — used by the AI session helpers
+                (save_claude_session / save_antigravity_session) and the
+                paru/yay log cleaner. Note: uv does not provide python3 on
+                PATH, and Arch's base does not include it, so it is listed
+                separately. All consumers degrade gracefully without it.
 
 ## Install Methods