feat: rename to issue-core and add task ingestion endpoint

Renames the package, distribution, CLI alias, Makefile targets, and
working directory from issue-facade to issue-core, signalling its
role as the authoritative task lifecycle manager for the Coulomb org
(peer to activity-core, rules-core, project-core).

Adds POST /issues/ ingestion endpoint for activity-core's IssueSink,
under a new optional [api] extra. The endpoint is served by `issue
serve`, authenticates via the ISSUE_CORE_API_KEY env var (Bearer or
X-API-Key header), and routes the TaskSpec payload to the configured
default backend with full traceability metadata embedded in
sync_metadata.

- T01: Python package issue_tracker -> issue_core, dir rename
- T02: registered in state hub under custodian domain
- T03: INTENT.md (what it is, what it isn't, how it fits)
- T04: SCOPE.md (in/out-of-scope, integration boundaries)
- T05: POST /issues/ via FastAPI + Uvicorn, 9 unit tests
- T06: docs/nats-task-ingestion.md design stub

Closes ISSC-WP-0001.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

issue_core/api/app.py

@@ -0,0 +1,26 @@
+"""
+FastAPI application factory for issue-core.
+"""
+
+from fastapi import FastAPI
+
+from .. import __version__
+from .ingest import router as ingest_router
+
+
+def create_app() -> FastAPI:
+    app = FastAPI(
+        title="issue-core",
+        description=(
+            "Authoritative task lifecycle manager for the Coulomb org. "
+            "POST /issues/ is the ingestion surface for activity-core's IssueSink."
+        ),
+        version=__version__,
+    )
+    app.include_router(ingest_router)
+
+    @app.get("/healthz", tags=["meta"])
+    async def healthz() -> dict:
+        return {"status": "ok", "version": __version__}
+
+    return app