Refactor/engine redesign

[atterpac]

Refactor of the engine/process internals behind the existing config API, plus bug fixes,
runtime logging controls, consolidated examples, and test coverage (engine+process now
~91%).

Bug fixes

• StringtoConfigTOML parsed TOML with the YAML decoder — now uses toml.Decode.
• IgnoreGit was a no-op: .gitignore patterns were read into a map nothing consulted. Now
  actually applied in shouldIgnore.
• slog.Error calls passed a bare error as a key, failing go vet and blocking go test.
• NewEngineFromConfig installed no logger; TOML/YAML constructors mutated a throwaway
  config copy and discarded Slog.
• CLI: unset comma flags produced [""] ignore lists; an unknown config extension left a
  nil engine → panic.
• BackgroundStruct was never run — only its DelayNext was read.
• Removed os.Exit(1) from library code.
• Untracked ~11 MB of committed binaries.

Refactor

• Replaced os.Chdir-based process running with per-command cmd.Dir, removing the
  global-cwd race with the watcher.
• New supervisor lifecycle: Start / Reload / Shutdown with per-process contexts.
  Removed the Ctxs/Cancels side maps and the dead ctx.Done() calls.
• Collapsed ~420 lines of duplicated per-OS process code into a setProcessGroup /
  killProcessTree / shellInvocation shim.
• Real trailing-edge debounce (the last save in a burst wins).
• Commands now run through a shell (sh -c / cmd /C), so quotes/pipes/&& work.
• Deleted the dead ignoreMap machinery.

Behavior changes

• BackgroundStruct.Cmd now runs as a long-lived background process: started once, survives
  reloads, killed at shutdown.
• ExecList (CLI -e) now actually produces processes; REFRESH_EXEC marks the primary.

New

• Runtime logging: SetLogLevel / DisableLogs / EnableLogs, plus SetLogger wrapping a
  custom logger.
• Re-exported Execute, ExecuteType, type constants, and the KILL_STALE/REFRESH_EXEC
  markers from the engine package.

Removed

• tui/ package and the root main.go (entrypoint is cmd/refresh); dropped the
  charmbracelet deps.
• Deprecated NewEngine, AttachBackgroundCallback, and the unused BackgroundCallback
  field.
• The old example/ module; consolidated into examples/basic and examples/kitchen-sink.

Tests

• examples/kitchen-sink doubles as the end-to-end integration test: all execute types,
  ChangeDir, callback veto, ignore rules, BackgroundStruct, clean shutdown.
• Unit tests for config parsing, ExecList conversion, debounce/watcher, logging, shell
  execution, failure paths, and CLI flag mapping.
• CI now runs go vet + go test -race ./... on Go 1.25; bumped EOL action versions.

Notes

• cmd.Dir change and the supervisor rewrite are the highest-risk parts; verified with
  -race and a live reload smoke test (no leaked processes).
• Command parsing relies on the shell, so command strings are not portable between POSIX and
  Windows.