chore(agents): add Rust code quality rules and skill (#3144)

* docs: update security advisory lessons

* chore(agents): add Rust code quality rules and skill

Add rules derived from full-project code review (48 findings across
7 dimensions) to prevent recurring issues in agent-generated code.

AGENTS.md changes:
- crates/AGENTS.md: error type design, concurrency, recursion safety,
  type casting, test quality rules
- root AGENTS.md: serde safety, naming conventions
- crates/ecstore/AGENTS.md: allocation discipline, lock ordering,
  recursion safety, dead code policy
- crates/notify/AGENTS.md: lock ordering for runtime_view/facade

Skill changes:
- code-change-verification: add Rust-specific checks (unwrap, as cast,
  clone, lock order, recursion, error types, test assertions)
- security-advisory-lessons: add serde deserialization safety pattern
- NEW rust-code-quality: automated scan + manual review checklist

.agents/skills/rust-code-quality/SKILL.md

@@ -0,0 +1,112 @@
+---
+name: rust-code-quality
+description: Enforce Rust-specific code quality rules on every code change. Use before merge to catch unwrap abuse, silent truncation, unnecessary cloning, lock ordering violations, recursion risks, and error type anti-patterns.
+---
+
+# Rust Code Quality Gate
+
+Use this skill on every Rust code change to enforce quality rules that `cargo clippy` does not catch.
+
+## Quick Start
+
+1. Identify changed `.rs` files.
+2. Run automated checks on changed files.
+3. Run manual review checklist on the diff.
+4. Report findings; block merge if P0/P1 issues exist.
+
+## Automated Checks
+
+Run these on every changed `.rs` file (excluding test modules):
+
+```bash
+# 1. unwrap/expect in production code
+rg -n '\.unwrap\(\)|\.expect\(' <changed-files> | grep -v '#\[cfg(test)\]' | grep -v 'test' | grep -v 'bench'
+
+# 2. Silent type truncation via `as` cast
+rg -n ' as (u8|u16|u32|u64|usize|i8|i16|i32|i64|isize)\b' <changed-files>
+
+# 3. String as error type
+rg -n 'Result<.*String>' <changed-files> | grep -v test
+
+# 4. Box<dyn Error> in public APIs
+rg -n 'Box<dyn.*Error' <changed-files> | grep -v test
+
+# 5. println/eprintln in production
+rg -n 'println!\|eprintln!' <changed-files> | grep -v test
+
+# 6. Ordering::Relaxed usage (verify each is intentional)
+rg -n 'Ordering::Relaxed' <changed-files>
+```
+
+## Manual Review Checklist
+
+For every Rust code change, verify:
+
+### Error Handling
+- [ ] No `unwrap()` or `expect()` in production code without justification comment
+- [ ] No `Result<_, String>` in public API signatures
+- [ ] No `Box<dyn Error>` in public trait/struct methods
+- [ ] `Error::source()` is overridden when inner error is stored
+- [ ] Error messages are actionable (what failed, with what input)
+
+### Type Safety
+- [ ] No silent `as` truncation (negative→unsigned, large→small)
+- [ ] `try_into()` or explicit clamping used for numeric conversions
+- [ ] No `f64 as usize` without prior clamping
+
+### Concurrency
+- [ ] Lock acquisition order is documented when multiple locks are used
+- [ ] No `tokio::sync` write guards held across `.await` without bounded hold time
+- [ ] Concurrent counters use `compare_exchange` loops, not load-then-store
+- [ ] `std::sync::Mutex` in async context is held only briefly, never across `.await`
+
+### Memory and Performance
+- [ ] No `.clone()` on structs with >5 heap-allocated fields in hot paths
+- [ ] `HashMap::with_capacity()` / `Vec::with_capacity()` used when size is known
+- [ ] Large buffers wrapped in `Arc` rather than cloned
+- [ ] Temporary string computations use `&str` or `Cow<str>` instead of `String`
+
+### Recursion Safety
+- [ ] Recursive functions have a depth limit or use iterative traversal
+- [ ] Tree/cache traversals handle corrupted/cyclic input safely
+
+### Testing
+- [ ] Every test function has at least one `assert!`
+- [ ] Tests use `.expect("context")` not bare `.unwrap()`
+- [ ] No `println!`/`eprintln!` in production code (use `tracing`)
+
+### Serde
+- [ ] Structs from untrusted input have `#[serde(deny_unknown_fields)]`
+- [ ] `#[serde(default)]` not used on security-critical fields without validation
+
+### Code Hygiene
+- [ ] No `#![allow(dead_code)]` at crate root
+- [ ] No camelCase statics or Hungarian notation
+- [ ] New string literals don't duplicate existing constants
+
+## Severity Classification
+
+- **P0 (Block merge)**: `unwrap()` in request hot path, silent truncation on user input, lock ordering violation, recursion without depth limit
+- **P1 (Must fix)**: `Result<_, String>` in public API, unnecessary clone in hot path, `Box<dyn Error>` in trait method
+- **P2 (Should fix)**: Missing `assert!` in test, `println!` in production, missing `with_capacity`
+- **P3 (Nice to fix)**: Naming convention violation, missing doc comment, `as_ptr()` vs `Arc::ptr_eq`
+
+## Output Template
+
+```
+## Rust Code Quality Report
+
+### Automated Scan
+- unwrap/expect in production: N found
+- as casts: N found
+- String errors: N found
+- println/eprintln: N found
+
+### Findings
+- [P1] `path:line` — description
+  - Fix: ...
+  - Validation: ...
+
+### Verdict
+PASS / BLOCKED (list blocking findings)
+```