feat: add fuzzing targets for untrusted input parsers

Add cargo-fuzz infrastructure with 5 fuzz targets exercising
security-critical code paths:

- fuzz_safety_sanitizer: Aho-Corasick + regex injection detection
- fuzz_safety_validator: Input validation (length, encoding, patterns)
- fuzz_leak_detector: Secret leak scanning (API keys, tokens)
- fuzz_tool_params: Tool parameter JSON validation
- fuzz_config_env: TOML/JSON config parsing

Each target exercises real IronClaw business logic with invariant
assertions. Includes corpus directories and setup documentation.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

fuzz/README.md

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+# IronClaw Fuzz Targets
+
+Fuzz testing for security-critical input parsing paths using [cargo-fuzz](https://github.com/rust-fuzz/cargo-fuzz) (libFuzzer).
+
+## Targets
+
+| Target | What it exercises |
+|--------|-------------------|
+| `fuzz_safety_sanitizer` | Prompt injection pattern detection (Aho-Corasick + regex) |
+| `fuzz_safety_validator` | Input validation (length, encoding, forbidden patterns) |
+| `fuzz_leak_detector` | Secret leak detection (API keys, tokens, credentials) |
+| `fuzz_tool_params` | Tool parameter JSON validation pipeline |
+| `fuzz_config_env` | Configuration parsing (TOML, JSON) |
+
+## Setup
+
+```bash
+cargo install cargo-fuzz
+rustup install nightly
+```
+
+## Running
+
+```bash
+# Run a specific target (runs until stopped or crash found)
+cargo +nightly fuzz run fuzz_safety_sanitizer
+
+# Run with a time limit (5 minutes)
+cargo +nightly fuzz run fuzz_leak_detector -- -max_total_time=300
+
+# Run all targets for 60 seconds each
+for target in fuzz_safety_sanitizer fuzz_safety_validator fuzz_leak_detector fuzz_tool_params fuzz_config_env; do
+    echo "==> $target"
+    cargo +nightly fuzz run "$target" -- -max_total_time=60
+done
+```
+
+## Adding New Targets
+
+1. Create `fuzz/fuzz_targets/fuzz_<name>.rs` following the existing pattern
+2. Add a `[[bin]]` entry in `fuzz/Cargo.toml`
+3. Create `fuzz/corpus/fuzz_<name>/` for seed inputs
+4. Exercise real IronClaw code paths, not just generic serde