doc/9p.md: add feed file to namespace table and examples

doc/9p.md

@@ -64,6 +64,13 @@ for s in session/*/agent/*/prompt; do echo "run tests" > "$s"; done
 # Wait for all agents to finish
 for s in session/*/agent/*/statewait; do cat "$s" > /dev/null; done
 
+# Pipe git diffs into an observer's feed (dedup is automatic)
+while :; do git diff HEAD --; sleep 5; done | ollie-9p write session/myproj/agent/obs/feed
+
+# Wire an agent's state changes to an observer
+while :; do ollie-9p read session/myproj/agent/coder/statewait >/dev/null; git diff HEAD --; done \
+  | ollie-9p write session/myproj/agent/obs/feed
+
 # Grep all agent plans
 grep -r "TODO" session/*/agent/*/plan
 
@@ -83,8 +90,10 @@ with a 10-line shell script instead of a framework.
   per-fid: write a request, read back the result
 - **`ctl` files** — control operations (stop, kill, rename, compact) are writes
   to a control file, not method calls
-- **Blocking reads** — `statewait` blocks until state changes, replacing event
-  subscriptions with a simple `cat`
+- **Blocking reads** — `statewait` and `feed` block until their value changes,
+  replacing event subscriptions with a simple `cat` in a loop
+- **Change detection** — `feed` deduplicates on the read side: same data written
+  repeatedly never wakes the reader
 - **Multiplexing** — multiple clients mount the same server simultaneously,
   each with an independent view through per-fid state
 
@@ -145,6 +154,7 @@ echo '{"prompt":"explain recursion"}' | ollie-9p rdwr generate
 |------|------|-------------|
 | `prompt` | w | Queue a new turn (write prompt text) |
 | `fifo` | r/w | Prompt queue (write=enqueue, read=dequeue) |
+| `feed` | r/w (blocking) | Change-detecting input: write data, read blocks until value changes |
 | `chat` | r (streaming) | Filtered text output (markers stripped) |
 | `chat.raw` | r (streaming) | Full output with block markers |
 | `statewait` | r (blocking) | Blocks until state changes, returns new state |