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/approvals.md

@@ -586,8 +586,8 @@ updates the root agent itself.
 
 ## Root-agent policy
 
-The system prompt (`hive-ag3nt/prompts/system.md`, rendered via
-`hive_ag3nt::prompt::render`) is the **same for every agent**; what
+The system prompt (`hive-agent/prompts/system.md`, rendered by
+`hive-agent/src/prompt.rs`) is the **same for every agent**; what
 varies is which MCP tools are surfaced (gated by tool groups and
 capabilities in `agent.nix`). There is no `role:manager` block that
 renders only for the root agent. The root agent's approval-gating