hive-forge: validate --forge <label> charset up front

argus flagged (approving) that an unvalidated label could build a path
outside the state dir; mara called it out as a usability issue in its
own right, not just a low-risk security nit — a typo'd label should
give a precise 'not a valid label' error, not a confusing file-not-found
or an unexpected traversal.

Reject anything outside the plain lowercase+digits+hyphens charset
dashboard/extra_forges.rs already enforces on write, before touching
the filesystem at all.

hive-forge/src/client.rs

@@ -241,13 +241,24 @@ pub fn index(n: u64) -> Result<i64> {
 /// the exact same two files `dashboard/extra_forges.rs` writes, so the
 /// read side can't drift from the write side. An unknown label (either
 /// file missing) is a clear error listing the labels actually found in
-/// the state dir, not a raw file-not-found.
+/// the state dir, not a raw file-not-found. A label outside the plain
+/// identifier charset the dashboard accepts (e.g. a typo containing
+/// `/` or `..`) is rejected up front with the same charset spelled out,
+/// rather than silently building a nonsense/traversing path and
+/// surfacing a confusing file error later.
 fn resolve_credentials(forge_label: Option<&str>) -> Result<(String, String)> {
     let Some(label) = forge_label else {
         let base = std::env::var("HIVE_FORGE_URL").unwrap_or_else(|_| DEFAULT_URL.to_owned());
         let token = read_token().context("read forge-token")?;
         return Ok((base, token));
     };
+    if !is_plain_ident(label) {
+        bail!(
+            "hive-forge: invalid --forge label {label:?} — must be lowercase \
+             letters, digits, and hyphens only (same rule the dashboard's \
+             FORGES tab enforces)"
+        );
+    }
 
     let dir = state_dir();
     let token_path = dir.join(format!("forge-{label}-token"));
@@ -270,6 +281,19 @@ fn resolve_credentials(forge_label: Option<&str>) -> Result<(String, String)> {
     bail!("hive-forge: no such forge {label:?} — provisioned forges: {known}");
 }
 
+/// Plain-identifier check matching `dashboard/extra_forges.rs`'s
+/// `is_plain_ident` (itself matching hive-priv's `validate_name_chars`)
+/// — lowercase ascii + digits + hyphens only. Rejecting anything else
+/// up front (rather than just letting a weird label fail to resolve a
+/// file) turns a confusing "no such forge" surprise into a precise
+/// "that's not a valid label" one, and incidentally means a label like
+/// `../../etc` can't be used to build a path outside the state dir.
+fn is_plain_ident(s: &str) -> bool {
+    !s.is_empty()
+        && s.chars()
+            .all(|c| c.is_ascii_lowercase() || c.is_ascii_digit() || c == '-')
+}
+
 /// Sidecar shape `dashboard/extra_forges.rs` writes alongside each
 /// `forge-<label>-token` file: just the base URL, pinned to the same
 /// `base_url` JSON key the write side uses.