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()