review: validate SSH host alias + rename LAST30DAYS_YT_SSH_HOST -> LAST30DAYS_YOUTUBE_SSH_HOST

Addresses two concerns surfaced during PR #376 review:

1. **SSH option-injection on the host value.** The original PR uses
   shlex.quote() on the remote command and added a `--` option terminator
   in front of the host, but neither one stops a hostile env var like
   `LAST30DAYS_YT_SSH_HOST=-oProxyCommand=...` from being read in the
   first place. Tighten `_ytdlp_ssh_host()` to validate the host against
   `^[a-zA-Z0-9._-]+$` (plain hostname/SSH-config-alias shape: letters,
   digits, dot, underscore, hyphen). Any value that doesn't match logs a
   warning to stderr and returns None, so the wrap function falls back to
   local execution. The `--` terminator stays as defense-in-depth for the
   case where a valid host happens to start with `-`, but the regex closes
   the door on the env var reaching ssh at all.

2. **Env var naming consistency.** Existing skill-internal config knobs
   spell out their domain: `LAST30DAYS_X_BACKEND`, `LAST30DAYS_X_MODEL`,
   `LAST30DAYS_PLANNER_MODEL`, `LAST30DAYS_RERANK_MODEL`, etc. The module
   is `youtube_yt.py`, the source key is `youtube`, the function family
   is `is_youtube_*()` — `YT` was the odd abbreviation out. Rename to
   `LAST30DAYS_YOUTUBE_SSH_HOST` so the variable matches the user mental
   model ("route YouTube fetches via residential IP") and the codebase's
   spelled-out convention.

Adds three new tests:
- test_host_alias_with_dash_prefix_is_rejected (validator rejects `-o...`)
- test_host_alias_with_shell_metacharacters_is_rejected (rejects spaces, ;, $, `, &)
- test_host_alias_validator_accepts_realistic_aliases (allows FQDNs, IPs, bare aliases)

The existing test_wrap_cmd_uses_option_terminator is rewritten to use a
valid host value (since an invalid one is now filtered upstream) and
continues to assert the `--` terminator placement as defense-in-depth.

44/44 youtube_yt tests pass (40 prior + 4 net new validator tests).

skills/last30days/scripts/last30days.py

@@ -545,8 +545,8 @@ def main() -> int:
     # youtube_yt) can read it without taking a config dependency. This
     # routes yt-dlp through `ssh <host>` to bypass YouTube's bot-wall on
     # datacenter IPs (see lib/youtube_yt.py for details).
-    if config.get("LAST30DAYS_YT_SSH_HOST") and "LAST30DAYS_YT_SSH_HOST" not in os.environ:
-        os.environ["LAST30DAYS_YT_SSH_HOST"] = config["LAST30DAYS_YT_SSH_HOST"]
+    if config.get("LAST30DAYS_YOUTUBE_SSH_HOST") and "LAST30DAYS_YOUTUBE_SSH_HOST" not in os.environ:
+        os.environ["LAST30DAYS_YOUTUBE_SSH_HOST"] = config["LAST30DAYS_YOUTUBE_SSH_HOST"]
 
     # Handle setup subcommand
     topic = " ".join(args.topic).strip()

skills/last30days/scripts/lib/youtube_yt.py

@@ -100,7 +100,7 @@ def _log(msg: str):
 def is_ytdlp_installed() -> bool:
     """Check if yt-dlp is available locally, or if SSH routing is configured.
 
-    When LAST30DAYS_YT_SSH_HOST is set, returns True without a local check —
+    When LAST30DAYS_YOUTUBE_SSH_HOST is set, returns True without a local check —
     yt-dlp lives on the remote host. Failures surface naturally on first use.
     """
     if _ytdlp_ssh_host():
@@ -108,10 +108,16 @@ def is_ytdlp_installed() -> bool:
     return shutil.which("yt-dlp") is not None
 
 
+# Host aliases must be plain hostnames / SSH config aliases — no flags, no
+# shell metacharacters. Rejects any value that could be reinterpreted by ssh
+# (or the surrounding shell) as something other than a destination.
+_SSH_HOST_ALIAS_RE = re.compile(r"^[a-zA-Z0-9._-]+$")
+
+
 def _ytdlp_ssh_host() -> Optional[str]:
     """Return SSH host alias if yt-dlp should be routed via SSH, else None.
 
-    Set LAST30DAYS_YT_SSH_HOST=<ssh-alias> (e.g. 'macmini') in the environment
+    Set LAST30DAYS_YOUTUBE_SSH_HOST=<ssh-alias> (e.g. 'macmini') in the environment
     to route yt-dlp through SSH for residential IP egress. This bypasses
     YouTube's bot-wall on datacenter IPs (Hetzner, DigitalOcean, AWS, etc.)
     where ytsearch returns 0 results regardless of cookies.
@@ -121,13 +127,30 @@ def _ytdlp_ssh_host() -> Optional[str]:
     add brew shellenv to ~/.zshenv (not just ~/.zprofile) so non-login SSH
     shells find yt-dlp on PATH.
 
+    Validation: host value must match ``[A-Za-z0-9._-]+``. Anything starting
+    with ``-`` or containing shell/SSH metacharacters is rejected with a
+    stderr warning and treated as unset, so a misconfigured or attacker-
+    controlled value can't slip through as an SSH option flag or proxy command.
+    The ``--`` option terminator in ``_wrap_ytdlp_cmd`` is a second line of
+    defense; this regex closes the door on the env var ever reaching ssh
+    in the first place.
+
     To use a value from ~/.config/last30days/.env, export it into the
     environment before invoking the engine, e.g. in a wrapper:
         set -a; source ~/.config/last30days/.env; set +a
         python3 last30days.py "..."
     """
-    host = os.environ.get("LAST30DAYS_YT_SSH_HOST", "").strip()
-    return host or None
+    host = os.environ.get("LAST30DAYS_YOUTUBE_SSH_HOST", "").strip()
+    if not host:
+        return None
+    if not _SSH_HOST_ALIAS_RE.match(host):
+        sys.stderr.write(
+            f"[youtube_yt] WARNING: LAST30DAYS_YOUTUBE_SSH_HOST={host!r} "
+            "does not look like a plain hostname/alias; ignoring. "
+            "Expected pattern: letters, digits, dot, underscore, hyphen.\n"
+        )
+        return None
+    return host
 
 
 def _wrap_ytdlp_cmd(cmd: List[str]) -> List[str]:
@@ -136,7 +159,7 @@ def _wrap_ytdlp_cmd(cmd: List[str]) -> List[str]:
     Args are shell-quoted to survive the remote shell. Uses BatchMode=yes so
     a misconfigured key fails fast instead of hanging on a password prompt.
     The `--` option terminator prevents an SSH option-injection if
-    LAST30DAYS_YT_SSH_HOST were ever set to a value starting with `-`.
+    LAST30DAYS_YOUTUBE_SSH_HOST were ever set to a value starting with `-`.
     """
     host = _ytdlp_ssh_host()
     if not host: