feat: add opencode agents and skills for development workflows

New agents:
- architect: system design, module interactions, data flow
- debugger: crash/memory/thread debugging with ASan, TSan, valgrind, gdb
- security-auditor: TLS, input validation, buffer safety, crypto audit
- refactorer: DRY, separation of concerns, API simplification
- integrator: integration tests, CI/CD pipeline, end-to-end verification
- code-explainer: architecture walkthrough, code explanation

New skills:
- debug-workflow: structured debugging workflow
- refactor: code restructuring with test verification
- security-audit: full security review with checklist
- benchmark: performance benchmarking with multi-run medians
- release: version bump, tests, tagging

Improved existing:
- c-reviewer: added security checklist
- cmake-expert: added ASan/TSan/UBSan configs, ccache, cross-compilation
- perf-analyst: added perf/valgrind/gprof commands
- test-writer: added fuzzing harnesses, integration test patterns
- pr-build: added sanitizer build variants
- pr-review: added security review, performance impact assessment

.opencode/skills/debug-workflow/SKILL.md

@@ -0,0 +1,138 @@
+---
+name: debug-workflow
+description: Debugs crashes, memory errors, hangs, and logic bugs in FastSync using structured methodology. Use when the user says "debug X", "fix crash", "investigate failure", "there's a bug", or needs help diagnosing issues.
+---
+
+# Debug Workflow Skill
+
+Structured debugging for FastSync: reproduce → isolate → diagnose → fix → verify. This skill CAN edit files, build, and run tests.
+
+## Workflow
+
+### Step 1: Understand the Problem
+
+Ask or gather:
+- What's the symptom? (crash, hang, wrong output, valgrind error)
+- What command triggers it?
+- Is it deterministic or intermittent?
+- What's the environment? (OS, compiler, network conditions)
+
+### Step 2: Reproduce
+
+Build with debug info:
+```bash
+rm -rf build
+cmake -B build -S . -DCMAKE_BUILD_TYPE=Debug
+cmake --build build -j$(nproc)
+```
+
+Try to reproduce the issue with the exact command the user provides.
+
+### Step 3: Isolate with Sanitizers
+
+**Memory errors (first priority):**
+```bash
+rm -rf build
+cmake -B build -S . \
+  -DCMAKE_C_FLAGS="-fsanitize=address -fno-omit-frame-pointer -g" \
+  -DCMAKE_EXE_LINKER_FLAGS="-fsanitize=address"
+cmake --build build -j$(nproc)
+./build/tests
+# or run the failing command
+```
+
+**Thread errors:**
+```bash
+rm -rf build
+cmake -B build -S . \
+  -DCMAKE_C_FLAGS="-fsanitize=thread -g" \
+  -DCMAKE_EXE_LINKER_FLAGS="-fsanitize=thread"
+cmake --build build -j$(nproc)
+./build/tests
+```
+
+**Valgrind (if ASan doesn't find it):**
+```bash
+valgrind --leak-check=full --show-leak-kinds=all --track-origins=yes \
+  ./build/client --source-dir /tmp/src --dest-dir /tmp/dst --save-to-disk
+```
+
+### Step 4: GDB Analysis
+
+If the issue is a crash or hang:
+```bash
+gdb --args ./build/client [args...]
+(gdb) run
+# when it crashes:
+(gdb) bt full
+(gdb) info locals
+(gdb) print variable_name
+```
+
+For hangs:
+```bash
+# In another terminal:
+kill -SIGABRT <pid>  # generates core dump
+gdb ./build/client core
+(gdb) thread apply all bt
+```
+
+### Step 5: Read the Code
+
+Read the relevant source files around the crash/failure point. Look for:
+- Unchecked return values
+- Null pointer dereferences
+- Buffer overflows
+- Use-after-free
+- Race conditions
+- Incorrect protocol handling
+
+### Step 6: Diagnose Root Cause
+
+Identify the exact file:line and what's wrong. Common patterns:
+- `send_n_data` / `receive_n_data` return value not checked
+- `data_destroy()` called but pointer still used
+- Queue operation without mutex in threaded code
+- Partial read/write not handled
+- Integer overflow in size calculations
+
+### Step 7: Fix
+
+Apply the minimal fix. Don't refactor while debugging — one change at a time.
+
+### Step 8: Verify
+
+```bash
+# Rebuild and test
+cmake -B build -S . && cmake --build build -j$(nproc)
+./build/tests
+
+# If integration test needed
+python3 test.py
+
+# Re-run under sanitizer to confirm fix
+rm -rf build
+cmake -B build -S . -DCMAKE_C_FLAGS="-fsanitize=address -fno-omit-frame-pointer" \
+  -DCMAKE_EXE_LINKER_FLAGS="-fsanitize=address"
+cmake --build build -j$(nproc)
+# reproduce the original failing command
+```
+
+### Step 9: Report
+
+Print a summary:
+```
+=== DEBUG SUMMARY ===
+Symptom: <what was happening>
+Root cause: <file:line — what's wrong>
+Fix: <what was changed>
+Verification: <how it was confirmed fixed>
+```
+
+## Rules
+- DO edit source files to fix issues
+- DO rebuild and test after fixes
+- DON'T refactor while debugging — minimal changes only
+- DON'T change behavior beyond fixing the bug
+- PRESERVE existing code style
+- ALWAYS verify with `./build/tests` after changes