fix: add lib/yt-lib.sh to install.sh library list

The yt-lib.sh library was missing from the install script's library
list, causing pos-media-yt-subtitles (and other yt-* tools) to fail
with 'No such file or directory' when run from /usr/local/bin/.

This adds yt-lib.sh to the install list and updates documentation
to reflect the change.

DOC/AGENT_Context_Project.md

@@ -231,7 +231,7 @@ User runs: ./install.sh [--apps|--full|--feature|--dry-run|--force|--skip <phase
 │   └─ Copies bin/* → /usr/local/bin/ (chmod 755)
 │   └─ Copies lib/*.sh (common, flags, notify, registry, entertainment-lib,
 │      scheduler-lib, config-ui, user-timers-lib, entertainment-plugin-lib,
-│      usb-lib, share-lib, menu-lib) → /usr/local/bin/ (chmod 644)
+│      usb-lib, share-lib, menu-lib, yt-lib) → /usr/local/bin/ (chmod 644)
 │   └─ Copies x64_bin/* → /usr/local/bin/ on x86_64 (arm64_bin/ on aarch64)
 │   └─ [if --feature] Copies features/* → /usr/local/bin/ (asks before overwriting),
 │                      then sets the matching feature flag

DOC/SCRIPTS.md

@@ -45,7 +45,7 @@ The phases:
 | # | Phase | Script/action |
 |---|-------|----------------|
 | 1 | preinstall | `preinstall.sh` — apt packages + yt-dlp |
-| 2 | scripts | Copies `bin/*` → `/usr/local/bin/` (755), `lib/common.sh` + `lib/flags.sh` + `lib/notify.sh` + `lib/entertainment-lib.sh` + `lib/entertainment-plugin-lib.sh` + `lib/scheduler-lib.sh` + `lib/config-ui.sh` + `lib/user-timers-lib.sh` + `lib/usb-lib.sh` + `lib/share-lib.sh` + `lib/menu-lib.sh` + `lib/registry.sh` → `/usr/local/bin/` (644). Copies precompiled arch binaries from `x64_bin/` (or `arm64_bin/`) → `/usr/local/bin/`. With `--feature`: also installs `features/*` (see below) |
+| 2 | scripts | Copies `bin/*` → `/usr/local/bin/` (755), `lib/common.sh` + `lib/flags.sh` + `lib/notify.sh` + `lib/entertainment-lib.sh` + `lib/entertainment-plugin-lib.sh` + `lib/scheduler-lib.sh` + `lib/config-ui.sh` + `lib/user-timers-lib.sh` + `lib/usb-lib.sh` + `lib/share-lib.sh` + `lib/menu-lib.sh` + `lib/registry.sh` + `lib/yt-lib.sh` → `/usr/local/bin/` (644). Copies precompiled arch binaries from `x64_bin/` (or `arm64_bin/`) → `/usr/local/bin/`. With `--feature`: also installs `features/*` (see below) |
 | 3 | postinstall | `postinstall.sh` — PATH, completion, SSH keys, systemd |
 | 4 | scalepoint | Shallow-clones ScaleTail templates to `/usr/local/share/linux_post_install/scale-tail` |
 | 5 (opt) | apps | `apps/install.sh` when `--apps` (interactive) or `--full` (all, non-interactive) |
@@ -245,6 +245,13 @@ Sourced by `bin/pos-tree` (tree rendering incl. the `[deps: …]` annotations) a
 
 ---
 
+## lib/yt-lib.sh — shared YouTube helpers
+
+**File:** `lib/yt-lib.sh` (installed to `/usr/local/bin/yt-lib.sh`)
+**Purpose:** shared helpers for yt-dlp-based media tools (`pos media yt *`). Provides `yt_check_deps` (dependency validation for yt-dlp/ffmpeg), `yt_validate_url` (return-1 URL checker, never exits — callers prefix errors), `yt_echo_cmd` (dry-run command display), and `classify_url` (URL type classification). Sourced by `bin/pos-media-yt-mp3`, `bin/pos-media-yt-mp4`, `bin/pos-media-yt-grab`, and `bin/pos-media-yt-subtitles` via the standard fallback chain.
+
+---
+
 ## features/autostart.sh — boot-time feature
 
 **File:** `features/autostart.sh` (installed to `/usr/local/bin/autostart.sh` by `./install.sh --feature`)

install.sh

@@ -175,7 +175,7 @@ if should_run 2 scripts; then
     done
     lib_count=0
     lib_names=""
-    for lf in common.sh flags.sh notify.sh entertainment-lib.sh scheduler-lib.sh config-ui.sh user-timers-lib.sh entertainment-plugin-lib.sh usb-lib.sh share-lib.sh menu-lib.sh registry.sh; do
+    for lf in common.sh flags.sh notify.sh entertainment-lib.sh scheduler-lib.sh config-ui.sh user-timers-lib.sh entertainment-plugin-lib.sh usb-lib.sh share-lib.sh menu-lib.sh registry.sh yt-lib.sh; do
         run sudo install -m 644 "lib/$lf" "/usr/local/bin/$lf"
         lib_count=$((lib_count + 1))
         lib_names+="$lf "