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/web-ui/dashboard.md

@@ -657,7 +657,8 @@ is `https://{domain}/`. Both are derived from the env var
 objects) that the nix module writes into the c0re container
 environment. `cert_fingerprint` is null for CA-trusted (e.g.
 Let's Encrypt) peers and non-null to pin a self-signed cert.
-`parse_peer_hives()` in `dashboard.rs` converts each entry to the
+`parse_peer_hives()` in `hive-c0re/src/dashboard/state_snapshot.rs`
+converts each entry to the
 `PeerHiveView { name: domain, url: "https://domain/" }` shape the
 frontend reads.