refactor: Guido simplification + testing.md compliance pass

[aorumbayev]

Summary

• Dead-code excision in core: deleted unused private wrappers and the legacy dict API on kagan.core (get_backend, to_legacy_config, guidance_hints, _acp_startup_timeout_seconds, _friendly_startup_error_message, _acp_process_exit_hint); fixed fake asyncio.StreamReader inheritance in _ByteCountingStreamReader and JsonRpcObjectStreamReader; consolidated process termination through a shared terminate_process() helper.
• Sessions.run() split and async-ified DB mutation helpers.
• TUI cosmetics: settings, context_footer, workspace, kanban, chat — pure cleanups, no behavior change.
• task_screen.py split (760 lines) into task_screen.py + _task_chat.py + _task_review.py + _task_stream.py.
• Final acp aliases gone: deleted _acp_handshake_timeout_seconds / _friendly_acp_error_message shims in cli/chat/acp.py; callers use the public kagan.core functions directly.
• Test layout aligned with docs/internal/testing.md:
  • 15 unit tests moved tests/core/unit/ → tests/unit/core/
  • 4 widget/screen unit tests moved tests/tui/ → tests/unit/tui/
  • All unit-test files now carry pytest.mark.unit
  • RateLimitMiddleware._safe_log_key and macOS env-sanitization unit tests pulled out of behavioral suites into tests/unit/server/test_middleware.py and tests/unit/cli/test_env_sanitization.py
  • Deleted tests/core/unit/test_transitions.py (asserted on locally defined helpers; the real state machine is covered by the comprehensive tests/unit/core/test_transitions.py)
  • Replaced asyncio.sleep polling in test_chat_first_turn_invokes_title_generator with an asyncio.Event set by the title generator + a yield-only loop
  • Added parametrized error-classification coverage for friendly_acp_error_message

Test plan

• uv run poe lint
• uv run poe typecheck — 0 errors (74 suppressed)
• uv run pytest tests/unit/ — 945 passed, 1 skipped
• uv run pytest tests/core/ tests/mcp/ tests/integrations/ -m "not slow" — 1513 passed, 1 skipped, 1 xpassed
• uv run pytest tests/tui/ -m "not snapshot and not slow" — 147 passed, 1 skipped
• CI green across the matrix

[greptile-apps]

Greptile Summary

This PR is a broad refactoring and cleanup pass: dead private wrapper functions are removed, fake asyncio.StreamReader inheritance is replaced with explicit composition, DB mutation helpers are promoted from sync to async, Sessions.run() is split into _run_detached / _run_attached, and the 760-line task_screen.py is split into three focused mixin modules. The test suite is reorganised to match the layout documented in testing.md.

• Core cleanup: _terminate_acp_process / _terminate_stdio_process are consolidated into a shared terminate_process() helper; _ByteCountingStreamReader and JsonRpcObjectStreamReader drop asyncio.StreamReader inheritance and replace __getattr__ with explicit delegation stubs.
• Sessions async-ification: _update_session_pid, _mark_session_running, _complete_session, and _fail_session are promoted to async methods using _db_async, removing all asyncio.to_thread wrapping.
• Test hygiene: 19 unit tests moved to tests/unit/, pytest.mark.unit added to all unit files, asyncio.Event-driven polling replaces the unbounded sleep loop in test_chat_first_turn_invokes_title_generator.

Confidence Score: 5/5

Safe to merge — all changes are refactoring with no behaviour modification to the production path.

The diff consolidates three previously-duplicated process-termination helpers, promotes sync DB helpers to async, and replaces fake inheritance with explicit composition. Every changed call site has a corresponding test fix, and both previous-thread concerns are directly addressed. No correctness regressions were identified.

No files require special attention.

Important Files Changed

Filename	Overview
src/kagan/core/_subprocess.py	Adds terminate_process() as a shared helper consolidating three previously-duplicate process shutdown implementations.
src/kagan/core/_acp_streams.py	Drops fake asyncio.StreamReader inheritance; replaces getattr with explicit readline, readuntil, read, readexactly, at_eof stubs.
src/kagan/core/_sessions.py	DB mutation helpers converted from sync+asyncio.to_thread to native async; run() split into _run_detached / _run_attached with proper -> Session return annotation.
src/kagan/core/_agent.py	_ByteCountingStreamReader now uses composition instead of asyncio.StreamReader inheritance; getattr removed; all four read methods explicitly delegated.
src/kagan/core/_acp.py	Private alias wrappers removed; all three _terminate_acp_process call sites replaced with terminate_process().
tests/core/test_chat_lifecycle.py	Unbounded sleep poll replaced with asyncio.Event-driven synchronisation and asyncio.wait_for timeout guard.
.github/workflows/ci.yml	Unit test path updated from tests/core/unit/ to tests/unit/core/ to match the new test layout.
tests/unit/test_sessions_shutdown.py	Monkeypatched sync lambdas replaced with proper async functions to match the newly-async method signatures.

_{Reviews (2): Last reviewed commit: "refactor(core): address greptile review ..." | Re-trigger Greptile}

[aorumbayev]

@greptileai