Replace polling with fsnotify for real-time file watching

Use OS-native filesystem notifications (inotify/kqueue/FSEvents) via
fsnotify instead of polling JSONL files every 500ms. Falls back to
polling when fsnotify is unavailable (NFS, cross-FS WSL2). Debounces
rapid writes with a 50ms coalesce window. Adds 11 new tests covering
fsnotify integration, debounce behavior, and polling fallback.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: phiat

README.md

@@ -63,7 +63,7 @@ claude-esp
 | `-n`       | Start from newest (skip history, live only)   |
 | `-l`       | List recent sessions                          |
 | `-a`       | List active sessions (modified in last 5 min) |
-| `-p <ms>`  | Poll interval in milliseconds (default 500)   |
+| `-p <ms>`  | Poll interval in ms (fallback mode only, default 500) |
 | `-v`       | Show version                                  |
 | `-h`       | Show help                                     |
 
@@ -136,10 +136,12 @@ Background task outputs are stored in:
 The watcher:
 
 1. Discovers active sessions (modified in last 5 minutes)
-2. Polls JSONL files every 500ms for new content
-3. Parses JSON lines and extracts thinking/tool_use/tool_result
-4. Discovers background tasks and correlates them with spawning agents
-5. Renders them in a TUI with tree navigation and filtering
+2. Uses OS-native filesystem notifications ([fsnotify](https://github.com/fsnotify/fsnotify)) to detect file changes in real-time (inotify on Linux, kqueue/FSEvents on macOS)
+3. Falls back to polling (configurable with `-p`) on filesystems that don't support notifications (NFS, some cross-FS WSL2 setups)
+4. Debounces rapid writes (50ms window) to efficiently handle burst output
+5. Parses JSON lines and extracts thinking/tool_use/tool_result
+6. Discovers background tasks and correlates them with spawning agents
+7. Renders them in a TUI with tree navigation and filtering
 
 ## tmux Setup
 

go.mod

@@ -15,6 +15,7 @@ require (
 	github.com/charmbracelet/x/cellbuf v0.0.13-0.20250311204145-2c3ea96c31dd // indirect
 	github.com/charmbracelet/x/term v0.2.1 // indirect
 	github.com/erikgeiser/coninput v0.0.0-20211004153227-1c3628e74d0f // indirect
+	github.com/fsnotify/fsnotify v1.9.0 // indirect
 	github.com/lucasb-eyer/go-colorful v1.2.0 // indirect
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/mattn/go-localereader v0.0.1 // indirect

go.sum

@@ -16,6 +16,8 @@ github.com/charmbracelet/x/term v0.2.1 h1:AQeHeLZ1OqSXhrAWpYUtZyX1T3zVxfpZuEQMIQ
 github.com/charmbracelet/x/term v0.2.1/go.mod h1:oQ4enTYFV7QN4m0i9mzHrViD7TQKvNEEkHUMCmsxdUg=
 github.com/erikgeiser/coninput v0.0.0-20211004153227-1c3628e74d0f h1:Y/CXytFA4m6baUTXGLOoWe4PQhGxaX0KpnayAqC48p4=
 github.com/erikgeiser/coninput v0.0.0-20211004153227-1c3628e74d0f/go.mod h1:vw97MGsxSvLiUE2X8qFplwetxpGLQrlU1Q9AUEIzCaM=
+github.com/fsnotify/fsnotify v1.9.0 h1:2Ml+OJNzbYCTzsxtv8vKSFD9PbJjmhYF14k/jKC7S9k=
+github.com/fsnotify/fsnotify v1.9.0/go.mod h1:8jBTzvmWwFyi3Pb8djgCCO5IBqzKJ/Jwo8TRcHyHii0=
 github.com/lucasb-eyer/go-colorful v1.2.0 h1:1nnpGOrhyZZuNyfu1QjKiUICQ74+3FNCN69Aj6K7nkY=
 github.com/lucasb-eyer/go-colorful v1.2.0/go.mod h1:R4dSotOR9KMtayYi1e77YzuveK+i7ruzyGqttikkLy0=
 github.com/mattn/go-isatty v0.0.20 h1:xfD0iDuEKnDkl03q4limB+vH+GxLEtL/jb4xVJSWWEY=