fix: stop draining queue on /clear (remove SessionEnd shim) (#2136)

* fix: stop draining queue on /clear (and on every other SessionEnd)

The SessionEnd hook was wired to session-complete on Claude Code, Gemini
CLI, the transcripts processor, the OpenCode plugin, and OpenClaw. All of
those paths called POST /api/sessions/complete, which marked the session
completed and abandoned every still-pending observation in the queue.

So typing /clear (or logging out, or quitting) wiped in-flight work that
the worker was perfectly happy to keep processing on its own.

Removed the entire shim:
- Deleted SessionEnd hook block in plugin/hooks/hooks.json
- Deleted src/cli/handlers/session-complete.ts and its registry entry
- Deleted POST /api/sessions/complete route + Zod schema in SessionRoutes
- Removed call from transcripts processor handleSessionEnd
- Removed call from opencode-plugin session.deleted handler
- Removed Gemini SessionEnd → session-complete mapping
- Removed openclaw scheduleSessionComplete + completionDelayMs + timer state
- Updated tests + comments accordingly

Explicit user-initiated deletion (DELETE /api/sessions/:id and
POST /api/sessions/:sessionDbId/complete from the viewer UI) still works
via SessionCompletionHandler.completeByDbId — that's the only path that
should drain the queue.

The worker self-completes via its SDK-agent generator's finally-block, so
no external completion call is needed.

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

* docs: clarify opencode-plugin session.deleted is in-memory cleanup only

Greptile P2: file-level header still implied session.deleted called the
worker. Now it only cleans up the local contentSessionIdsByOpenCodeSessionId
map; worker self-completes via the SDK-agent generator finally-block.

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

---------

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

openclaw/src/index.test.ts

@@ -266,12 +266,6 @@ describe("Observation I/O event handlers", () => {
             return;
           }
 
-          if (req.url === "/api/sessions/complete") {
-            res.writeHead(200, { "Content-Type": "application/json" });
-            res.end(JSON.stringify({ status: "completed" }));
-            return;
-          }
-
           if (req.url?.startsWith("/api/context/inject")) {
             res.writeHead(200, { "Content-Type": "text/plain; charset=utf-8" });
             res.end("# Claude-Mem Context\n\n## Timeline\n- Session 1: Did some work");
@@ -446,8 +440,7 @@ describe("Observation I/O event handlers", () => {
     assert.ok(summarizeRequest!.body.contentSessionId.startsWith("openclaw-summarize-test-"));
 
     const completeRequest = receivedRequests.find((r) => r.url === "/api/sessions/complete");
-    assert.ok(completeRequest, "should send complete to worker");
-    assert.ok(completeRequest!.body.contentSessionId.startsWith("openclaw-summarize-test-"));
+    assert.ok(!completeRequest, "should not send complete (worker self-completes)");
   });
 
   it("agent_end extracts text from array content", async () => {