Wave 5+6: plugin ABI hardening, build modernization, ABI/security docs

Wave 5 (9 parallel agents):
- W1.1 atomic diag callback + DLL handle release on shutdown (lin)
- W2.1 unify cross-DLL heap discipline (host->alloc/free/strdup) (chen)
- W2.2 secure_zero api_key on shutdown for deepseek/anthropic (cao)
- W3 CMake modernization: target-based cxx_std_20, dstalk_boost_config
  INTERFACE lib, root-level RUNTIME_OUTPUT_DIRECTORY (hu)
- W4 GitHub Actions CI with dynamic Linux/Windows matrix (ma)
- W5.1 SSE buffer_body to cut peak memory ~67% on 32K streams (zhou)
- W6.1 LSP JSON-RPC frame parser hardened against header reordering (sun)
- W7 smoke test: copy plugin DLLs post-build + Boost.JSON src.hpp fix
  for full 9-plugin load coverage (wang)
- W8.1 README slimmed 398->92, Diataxis docs/ skeleton (deng)

Wave 6 (6 parallel agents):
- W9.1 docs/explanation: architecture + plugin-lifecycle (deng)
- W9.3 log credential leak audit (0 vulns, audit trail in
  docs/explanation/security-logging.md) (cao)
- W9.4 docs/reference/plugin-abi.md - 7-point ABI contract (lin)
- W9.6 CLI /history command + status integration (zhao)
- W9.8 plugin_loader fault tolerance: per-plugin failure no longer
  aborts dstalk_init (huang)
- W9.10 host_api unit tests: tests/host_api_test.cpp, 8 cases (liu)

CEO oversight (preexisting bugs fixed during Wave 5 verification):
- lsp_plugin.cpp:449 forward decl mismatch (int vs void)
- tools_plugin.cpp:109 missing forward decl

Multi-agent collaboration framework:
- agents/WORKFLOW.md: 6-stage protocol, two-tier governance,
  prompt template, technical constraints registry

Build: cmake --build 0 error / 0 warning. Tests: 2/2 100% pass.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tests/smoke_test.cpp

@@ -68,7 +68,7 @@ int main()
         char* content = nullptr;
         if (file_io->read(file_path.string().c_str(), &content) == 0 && content) {
             bool ok = std::strcmp(content, sample_content) == 0;
-            std::free(content);
+            dstalk_free(content);
             if (ok) {
                 std::cout << "[OK] file_io->read content matches\n";
             } else {
@@ -123,7 +123,7 @@ int main()
             char* saved = nullptr;
             if (file_io->read(saved_path.string().c_str(), &saved) == 0 && saved) {
                 bool session_ok = std::strcmp(saved, session_content) == 0;
-                std::free(saved);
+                dstalk_free(saved);
                 if (session_ok) {
                     std::cout << "[OK] session content matches after save/load\n";
                 } else {
@@ -331,7 +331,7 @@ int main()
                               << "  expected length: " << std::strlen(escape_content) << "\n"
                               << "  got length: " << std::strlen(read_back) << "\n";
                 }
-                std::free(read_back);
+                dstalk_free(read_back);
             } else {
                 std::cerr << "[FAIL] escape content read-back failed\n";
             }