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/tests/test_build.py

@@ -70,6 +70,38 @@ def test_portable_bundle_is_conformant_and_self_contained(tmp_path: Path) -> Non
     assert not any(path.is_symlink() for path in root.rglob("*"))
 
 
+def _harness_identity(root: Path) -> dict[str, str]:
+    """Read the generated core/_harness_id.py without importing it."""
+    values: dict[str, str] = {}
+    for line in (root / "core" / "_harness_id.py").read_text(encoding="utf-8").splitlines():
+        if "=" in line and not line.lstrip().startswith("#"):
+            name, _, raw = line.partition("=")
+            values[name.strip()] = raw.strip().strip('"')
+    return values
+
+
+def test_the_portable_bundle_declares_no_host_application(tmp_path: Path) -> None:
+    """It runs in whatever editor a user drops it into, so it cannot know the host.
+
+    X-Application is allowlisted server-side. A guessed value is silently dropped
+    there, which is the worst outcome: the wire says we know the host and the
+    stored event says we do not.
+    """
+    identity = _harness_identity(build("mem0-agent-plugin", "portable", tmp_path / "portable"))
+
+    assert identity["PLATFORM_APPLICATION"] == ""
+    # The PostHog-side label is still useful for grouping and stays populated.
+    assert identity["HARNESS_ID"] == "coding-agent"
+    assert identity["PLATFORM_SOURCE"] == "MEM0_PLUGIN"
+
+
+@pytest.mark.parametrize("host", ["claude-code", "cursor", "codex", "kimi", "antigravity"])
+def test_a_native_bundle_names_the_host_it_was_built_for(host: str, tmp_path: Path) -> None:
+    identity = _harness_identity(build(host, "native", tmp_path / host))
+
+    assert identity["PLATFORM_APPLICATION"] == host
+
+
 @pytest.mark.parametrize("host", ["claude-code", "cursor", "codex", "kimi", "antigravity"])
 def test_native_bundle_is_self_contained(host: str, tmp_path: Path) -> None:
     root = build(host, "native", tmp_path / host)