Collector milestone 2: source interface + drop-folder source

source.Source is the contract every Collector input implements: Name +
Run(ctx, emit). Sources don't own state — they convert external events
into emit calls. Dispatcher routes.

internal/source/dropfolder: watches ~/Nyx/workspace/incoming/ for
*.json drop files. fsnotify-driven with periodic poll fallback (default
30s safety net for missed events). Each file:

1. Parsed against the spec §3.1.2 schema with DisallowUnknownFields.
2. Valid → emitted, then file deleted.
3. Invalid (missing fields, bad type/priority, unknown fields, garbage)
   → moved to .dead-letter/ with a sidecar .reason file for forensics.
4. Emit failure → file retained in place for retry (transient errors
   shouldn't be permanent dead-letters).

Also: initial-scan on Run() drains files that landed before the watcher
attached, catching up after a Collector restart.

14 tests in the package — schema validation table for all error cases,
initial-scan, live inotify drop, post-emit delete, dead-letter +
sidecar, emit-failure retention. Plus the 7 inbox tests still passing.

Pinned fsnotify v1.7.0 (Go 1.22 compatible; 1.10.x demanded toolchain
1.23 which isn't in apt yet). go.mod stays at 1.22 to match VPS.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

go.mod

@@ -1,3 +1,7 @@
 module git.botbought.ai/foreman/agent-watcher
 
-go 1.22.2
+go 1.22
+
+require github.com/fsnotify/fsnotify v1.7.0
+
+require golang.org/x/sys v0.13.0 // indirect