feat: pos media grab — smart URL classifier for auto-download

New tool that classifies URLs by domain and delegates to pos media mp3
(audio) or pos media mp4 (video). Listener gains URL detection step
between prefix map and AI bridge — bare URLs auto-download.

Domain rules: music.youtube.com/soundcloud/bandcamp → mp3,
youtube/vimeo/twitch → mp4 --best, unknown → configurable default.

28 test cases / 70 assertions / 0 failed.
make gen && make check green, make lint 0 FAIL / 0 WARN.

completions/pos.bash

@@ -9,6 +9,7 @@ _pos_flags[communication-telegram-sender]="--type --caption --parse-mode --no-pr
 _pos_flags[docker-stack]="-a --all"
 _pos_flags[docker-vbox]="--dir --gpu --device --port --cpus --memory --network"
 _pos_flags[entertainment-send]="--print --markdown"
+_pos_flags[media-grab]="--audio --video --best --worst --output --no-playlist --cookies --dry-run"
 _pos_flags[media-mp3]="--output --no-playlist --cookies --by-artist --dry-run"
 _pos_flags[media-mp4]="--format --best --worst --output --no-playlist --cookies --dry-run"
 _pos_flags[media-sync]="--mp3 --mp4 --source --dry-run"
@@ -46,7 +47,7 @@ _pos_subcmds[system-schedule]="run list config enable disable status migrate men
 _pos_subcmds[ai]="ask chat sessions capture models providers alias gemini openrouter"
 # GEN:END possubcmds
 # GEN:START posconfigscopes
-declare -a _pos_config_scopes=(ai compose entertainment matrix notify scrcpy system telegram ytsync)
+declare -a _pos_config_scopes=(ai compose entertainment grab matrix notify scrcpy system telegram ytsync)
 # GEN:END posconfigscopes
 
 _pos() {