Implement inbox read cursors for unread threads

docs/inbox-cli.md

@@ -39,6 +39,7 @@ Those decisions belong to `orch`.
 - `lease`: an exclusive worker claim for a thread
 - `artifact`: a path or file reference attached to a message
 - `event`: a monotonic record used to wake blocking waiters
+- `read cursor`: the last thread message an agent has explicitly consumed
 
 ## Required Fields
 
@@ -161,6 +162,8 @@ Suggested flags:
 - `--limit N`
 - `--unread`
 
+`--unread` should use a per-agent thread read cursor, not a latest-message heuristic alone.
+
 ### `inbox claim`
 
 Acquire a lease on a thread.
@@ -280,6 +283,7 @@ Suggested flags:
 
 - `--thread THREAD_ID`
 - `--json`
+- `--mark-read`
 
 `show` should include per-message artifact references when present.
 
@@ -420,6 +424,16 @@ CREATE TABLE IF NOT EXISTS artifacts (
   FOREIGN KEY(message_id) REFERENCES messages(message_id)
 );
 
+CREATE TABLE IF NOT EXISTS thread_reads (
+  thread_id TEXT NOT NULL,
+  agent_id TEXT NOT NULL,
+  last_read_message_id TEXT NOT NULL,
+  last_read_at TEXT NOT NULL,
+  PRIMARY KEY(thread_id, agent_id),
+  FOREIGN KEY(thread_id) REFERENCES threads(thread_id),
+  FOREIGN KEY(last_read_message_id) REFERENCES messages(message_id)
+);
+
 CREATE TABLE IF NOT EXISTS events (
   event_id INTEGER PRIMARY KEY AUTOINCREMENT,
   run_id TEXT NOT NULL,
@@ -441,6 +455,9 @@ CREATE INDEX IF NOT EXISTS idx_messages_thread_created
 
 CREATE INDEX IF NOT EXISTS idx_events_thread_event
   ON events(thread_id, event_id);
+
+CREATE INDEX IF NOT EXISTS idx_thread_reads_agent
+  ON thread_reads(agent_id, last_read_at);
 ```
 
 ## Concurrency Notes