docs: fix remaining stale internal references from the #3281 sweep

Two follow-ups the per-file sweep workers flagged but couldn't fix
themselves (either not their assignment, or not visible from their
worktree):

- web-ui/dashboard.md's P33RS tab named dashboard.rs, which no longer
  exists (hive-c0re/src/dashboard/ is a module dir now); the function
  moved to state_snapshot.rs.
- approvals.md and conventions.md used hive_ag3nt:: as a Rust
  module-path prefix. hive-agent is a bin-only crate (no lib target),
  so that prefix never resolved to anything; also fixed the
  hive-ag3nt/prompts/system.md path to the real hive-agent/prompts/.

All three verified directly against the current source tree, not
guessed.

docs/conventions.md

@@ -16,7 +16,7 @@ exist because something already went wrong without them.
 ## Hive identity (label + domain + display names)
 
 Four env vars cover the identity surface, read by
-`hive_ag3nt::identity`:
+`hive-agent/src/identity.rs`:
 
 - `HIVE_LABEL` — short, hive-local agent label (`iris`,
   `damocles`). `label()` returns it; falls back to empty string if