refactor: complexity audit -- eliminate god-method forks, O(n^2) loops, redundant I/O

CHANGELOG.md

@@ -8,6 +8,15 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Changed
+
+- Rename `DownloadStrategyManager` to `DownloadDelegate` to better reflect Facade/Delegate pattern (#918)
+- Fix incorrect double-checked locking in marketplace registry `_load()` -- hold lock across full check+read+set (#918)
+
+### Fixed
+
+- Remove redundant `seen` set from `_scan_patterns()` discovery walk (#918)
+
 ## [0.10.0] - 2026-04-27
 
 ### Added
@@ -24,12 +33,47 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - Docs site auto-deploys again after bot-cut releases (now triggered on tag push). (#981)
 
+### Documentation
+
+- Clarify `NullCommandLogger` partial interface and visible-output semantics in docstring (#918)
+
 ### Maintainer tooling
 
 - `pr-description-skill` ships an evals suite so PR-description quality regressions are caught in CI without an LLM API key. (#985)
 - `pr-description-skill` mermaid guidance hardened with `assets/mermaid-conventions.md` (diagram-type-by-intent + GitHub-renderer gotchas `mmdc` misses). (#984)
 - Cowork tests mock `sys.platform` so the macOS auto-detection paths don't false-fail on Windows CI. (#989)
 
+### Added
+
+- `NullCommandLogger` class (`src/apm_cli/core/null_logger.py`) -- null-object pattern for logger injection, eliminating 32 conditional logger forks in `MCPIntegrator`. (#918)
+- Thread-safety infrastructure: `_get_console()` double-checked locking singleton, marketplace registry cache `threading.Lock`. (#918)
+- 40 characterisation tests for `MCPIntegrator` methods (`install()`, `remove_stale()`, `collect_transitive()`). (#918)
+- `_build_children_index()` helper in uninstall engine for O(n) reverse-dependency lookups. (#918)
+- Performance benchmarks and scaling guards for complexity audit refactors (`tests/benchmarks/test_audit_benchmarks.py`, `test_scaling_guards.py`): 16 benchmark tests covering dependency parsing, children index, primitive discovery, registry cache, console singleton, and NullCommandLogger; 3 scaling-ratio guards run in the default test suite to catch O(n^2) regressions. (#918)
+- Expanded performance benchmark suite with P0 and P1 hot-path coverage: `compute_package_hash`, `get_all_dependencies`, `is_semantically_equivalent`, `flatten_dependencies`, `to_yaml`, `compute_deployed_hashes`, `optimize_instruction_placement`, `_rewrite_markdown_links`, `partition_managed_files`, LockFile round-trip, and `register_contexts` -- 52 new benchmark tests plus 2 additional scaling guards (#918)
+- Iteration 2 benchmark coverage: `_match_double_star` recursive glob matcher, `ContentScanner.scan_text` and `strip_dangerous` security scanning, `build_dependency_tree` BFS resolver, `_parse_ls_remote_output` and `_sort_remote_refs` git ref parsing, `analyze_directory_structure` compiler analysis, and `collect_transitive` MCP integration -- 77 new benchmark tests plus 1 additional scaling guard (#918)
+
+### Changed
+
+- `MCPIntegrator` logger handling: methods default to `NullCommandLogger` instead of `None`, removing 32 `if logger:` / `elif logger:` conditional forks (net -91 production lines). (#918)
+- Install pipeline lockfile reads reduced from 2x to 1x by caching early lockfile on `InstallContext`. (#918)
+- `APMPackage.from_apm_yml()`: deduplicated dependency parsing via `_parse_dependency_dict()` classmethod. (#918)
+- Uninstall engine BFS orphan detection: O(n^2) full-scan replaced with O(n) reverse-dep index. (#918)
+- Primitive discovery scanning: 9+ `glob.glob()` calls replaced with single `os.walk` + `fnmatch` pass. (#918)
+- MCP registry config reads: O(servers x runtimes) reduced to O(runtimes) via function-scoped cache. (#918)
+- `_get_console()`: returns thread-safe singleton instead of creating new `Console()` per call. (#918)
+- Marketplace registry cache: `_load()`, `_save()`, `_invalidate_cache()` protected with `threading.Lock`. (#918)
+- Complexity audit -- decomposed god functions in `reference.py`, `audit.py`, `deps/cli.py`, and `script_runner.py` into focused single-responsibility helpers (largest: `audit()` 290 lines split into thin dispatcher + `_audit_ci_gate` + `_audit_content_scan` with shared `_AuditConfig` dataclass). (#918)
+- Decomposed `github_downloader.py` into three modules: `git_remote_ops.py` (ref parsing), `download_strategies.py` (backend downloads), and a slimmed orchestrator (#918)
+- Decomposed `install()` god function (555 lines) into focused helpers with `InstallContext` parameter bundle (#918)
+
+### Fixed
+
+- Bare `except:` clauses in `formatters.py` (5) and `script_formatters.py` (2) now catch `Exception` instead of `BaseException`, allowing `KeyboardInterrupt` and `SystemExit` to propagate correctly. (#918)
+- Silent auth fallback in `discovery.py:_get_token_for_host()` now logs `logger.debug()` when the token manager fails, making credential resolution failures visible with `--verbose`. (#918)
+- Silent `except Exception: pass` handlers in `agents_compiler.py` (3) now emit `_logger.debug()` traces for config loading and constitution injection failures. (#918)
+- Double `iterdir()` walk in `script_runner.py:_resolve_prompt_file()` collapsed to a single pass. (#918)
+
 ## [0.9.4] - 2026-04-27
 
 ### Added