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/claude-code-plugin/core/_harness_id.py

@@ -5,5 +5,7 @@ SOURCE_TAG = "CLAUDE_CODE_PLUGIN"
 
 # Platform-side vocabulary (mem0_event.source + X-Application). The whole
 # plugin family is one source; which editor it runs in is the application.
+# An empty application means the host is unknown, and memory_core omits
+# the header entirely rather than sending a placeholder.
 PLATFORM_SOURCE = "MEM0_PLUGIN"
 PLATFORM_APPLICATION = "claude-code"