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)

functions/_fish_deps_catalog.fish

@@ -16,27 +16,27 @@ function _fish_deps_catalog
     set -g _fdc_bins \
         uv cargo fish fisher starship fzf zoxide direnv paru yay \
         wakatime tailscale \
-        eza lsd bat btop dust duf prettyping ov rg lazygit lazydocker trash kitty wezterm
+        eza lsd bat btop dust duf prettyping ov rg lazygit lazydocker trash kitty wezterm python3
 
     set -g _fdc_tiers \
         req req req req req req req req rec rec \
         int int \
-        rec rec rec rec rec rec rec rec rec rec rec rec rec rec
+        rec rec rec rec rec rec rec rec rec rec rec rec rec rec rec
 
     set -g _fdc_cargo \
         "" "" "" "" starship "" zoxide "" "" "" \
         "" "" \
-        eza lsd bat "" du-dust "" "" ov ripgrep "" "" trashy "" ""
+        eza lsd bat "" du-dust "" "" ov ripgrep "" "" trashy "" "" ""
 
     set -g _fdc_pm \
         uv cargo fish "" starship fzf zoxide direnv "" yay \
         wakatime tailscale \
-        eza lsd bat btop dust duf prettyping ov ripgrep lazygit lazydocker trash kitty wezterm
+        eza lsd bat btop dust duf prettyping ov ripgrep lazygit lazydocker trash kitty wezterm python
 
     set -g _fdc_special \
         curl-uv rustup-installer git-cargo-fish fisher-bootstrap curl-installer fzf-update "" "" paru-build yay-build \
         wakatime-binary "" \
-        "" "" "" "" "" "" "" "" "" "" curl-lazydocker "" "" ""
+        "" "" "" "" "" "" "" "" "" "" curl-lazydocker "" "" "" ""
 end
 
 # SYNOPSIS

functions/save_antigravity_session.fish

@@ -7,6 +7,12 @@
 set -l input (cat)
 
 # 2. Extract session_id using Python
+#    If python3 is unavailable, emit valid empty JSON and skip session
+#    tracking rather than erroring (see AGENTS.md Convention §6).
+if not type -q python3
+    echo '{}'
+    exit 0
+end
 set -l sid (echo $input | python3 -c "import sys,json; print(json.load(sys.stdin).get('session_id', ''))")
 set -l session_file ".antigravity_session"
 

functions/save_claude_session.fish

@@ -4,7 +4,14 @@
 # SPDX-License-Identifier: AGPL-3.0-or-later
 
 # 1. Read input and extract session ID
+#    python3 parses the session_id out of the hook's JSON payload. If it is
+#    unavailable we emit valid empty JSON and skip session tracking rather
+#    than erroring (see AGENTS.md Convention §6).
 set -l input (cat)
+if not type -q python3
+    echo '{}'
+    exit 0
+end
 set -l sid (echo $input | python3 -c "import sys,json; print(json.load(sys.stdin).get('session_id', ''))")
 set -l session_file ".claude_session"