Add integration-style tests with fake LLM clients for agent dispatch and
llmloop runner, plus new unit test files for gitcmd, session/history,
tool/code_comment, tool/filereader_read, and viewer/store packages.
Replace os.ReadFile + strings.Split with bufio.Reader streaming in
FileReadProvider, so only the requested window (up to 500 lines) is kept
in memory instead of the entire file. Adds Runner.Stream for semaphore-
aware stdout piping, FileReader.ReadLines for line-windowed reading, and
validates invalid line ranges. Reduces per-goroutine memory from ~8MB to
~44KB for large files under concurrent workloads.
Introduce gitcmd.Runner with channel-based semaphore to cap concurrent
git subprocesses (default 16, configurable via --max-git-procs). Route
all tool-layer and diff-layer git calls through the shared runner.
Also fix diff.Provider.runGit lacking context.Context — now the full
chain (GetDiff → MergeBase → ParseDiffText → finalizeDiff) propagates
the caller's context for proper cancellation and timeout support.
