feat(ci): add Criterion performance benchmarks for hot paths (#638)

Add benchmarks using Criterion for:
- XML tool-call parsing (single and multi-call)
- Native tool-call parsing
- SQLite memory store/recall/count operations
- Full agent turn cycle (text-only and with tool call)

Add CI workflow (.github/workflows/benchmarks.yml) that:
- Runs benchmarks on push to main and on PRs
- Uploads Criterion results as artifacts
- Posts benchmark summary as PR comment for regression visibility

Ref: https://github.com/zeroclaw-labs/zeroclaw/issues/618 (item 7)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Cargo.toml

@@ -171,3 +171,8 @@ panic = "abort"
 [dev-dependencies]
 tokio-test = "0.4"
 tempfile = "3.14"
+criterion = { version = "0.5", features = ["async_tokio"] }
+
+[[bench]]
+name = "agent_benchmarks"
+harness = false