fix(plugins): do not claim a host application the portable bundle cannot know

Review point. The portable bundle is built with host "coding-agent", and the
build wrote that straight into PLATFORM_APPLICATION, so every portable install
sent X-Application: coding-agent.

That value is not in the platform's allowlist, so it was already being dropped
server-side. The effect was the worst of both: the wire claimed we knew the
editor, the stored event recorded that we did not, and nothing said which was
right. An absent header says the same thing honestly and costs a lookup.

HARNESS_ID stays "coding-agent". It is the PostHog-side label, it is true, and
grouping portable installs together there is useful.

Native bundles are unchanged apart from the regenerated comment. Covered by two
new build tests: portable declares no application, and each native names the
host it was generated for.

Claude-Session: https://claude.ai/code/session_01C7tEmH86HAr7GoAAKCEHZb

integrations/agent-plugin-core/build/build.py

@@ -81,15 +81,23 @@ def replace_output(staged: Path, output: Path) -> Path:
     return output
 
 
-def _render_harness_id(host: str) -> str:
+def _render_harness_id(host: str, *, portable: bool = False) -> str:
     """Emit core/_harness_id.py for one host.
 
     Carries both vocabularies from a single definition: the PostHog `source` tag
     and the platform's X-Mem0-Source / X-Application pair. Keeping them together
     is what stops the two from drifting into separate vocabularies for the same
     thing.
+
+    The portable bundle runs in whatever editor a user drops it into, so it does
+    not know its host and must not guess one. HARNESS_ID stays "coding-agent",
+    which is true and useful for grouping in PostHog, but PLATFORM_APPLICATION is
+    left empty: X-Application names a real host app, is checked against an
+    allowlist server-side, and a value that is always discarded is worse than no
+    value -- it reads like an attribution we have and do not.
     """
     tag = host.upper().replace("-", "_") + "_PLUGIN"
+    application = "" if portable else host
     return (
         '"""Generated by integrations/agent-plugin-core/build/build.py. Do not edit."""\n'
         "\n"
@@ -98,8 +106,10 @@ def _render_harness_id(host: str) -> str:
         "\n"
         "# Platform-side vocabulary (mem0_event.source + X-Application). The whole\n"
         "# plugin family is one source; which editor it runs in is the application.\n"
+        "# An empty application means the host is unknown, and memory_core omits\n"
+        "# the header entirely rather than sending a placeholder.\n"
         'PLATFORM_SOURCE = "MEM0_PLUGIN"\n'
-        f'PLATFORM_APPLICATION = "{host}"\n'
+        f'PLATFORM_APPLICATION = "{application}"\n'
     )
 
 
@@ -122,7 +132,7 @@ def _bundle_python(
     # to call telemetry.init(). mcp_server.py and the detached telemetry.py sender
     # never did, which is how MCP searches reported harness=generic and every
     # batch they drained was labelled MEM0_PLUGIN regardless of the real host.
-    (core / "_harness_id.py").write_text(_render_harness_id(host), encoding="utf-8")
+    (core / "_harness_id.py").write_text(_render_harness_id(host, portable=portable), encoding="utf-8")
 
     values = {
         "PLUGIN_ROOT": plugin_root,