Address Bob's notes 2 and 3 with documenting comments

Note 2: poll_fallback_seconds==0 silently means 'use default'. Document
in example yaml; no v1 way to disable polling, and we don't think
anyone needs that.

Note 3: Writer.locks grows unbounded. Bounded in practice (<10 agents);
add a comment for the future maintainer who may need to evict.

Notes 1 and 4 left unchanged: missingkey=zero is the friendlier choice
(produces a visible '<no value>' in the inbox rather than a silent
500); fsnotify double-fire is already handled by os.ErrNotExist on
second-read.

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

internal/inbox/inbox.go

@@ -38,6 +38,11 @@ type Event struct {
 //
 // Concurrency: a single Writer is safe for use from multiple goroutines.
 // One sync.Mutex per recipient bounds contention to per-file granularity.
+//
+// The locks map grows monotonically as new recipients appear. In practice
+// the agent network has <10 recipients, so the leak is bounded; if this
+// daemon ever lives in a system with thousands of recipients, replace the
+// map with an LRU or shard-and-evict scheme.
 type Writer struct {
 	dir string