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/schemas.py

@@ -0,0 +1,50 @@
+"""
+Pydantic schemas for the issue-core REST API.
+
+The TaskIngestionRequest schema matches activity-core's IssueSink TaskSpec
+payload exactly. See:
+  - SCOPE.md "TaskSpec payload" section
+  - activity-core docs/adr/adr-001-event-bridge-architecture.md
+"""
+
+from typing import List, Literal, Optional
+from uuid import UUID
+
+from pydantic import BaseModel, ConfigDict, Field
+
+
+SourceType = Literal["rule", "instruction"]
+Priority = Literal["high", "medium", "low"]
+BackendName = Literal["gitea", "sqlite", "github"]
+
+
+class TaskIngestionRequest(BaseModel):
+    """TaskSpec payload from activity-core's IssueSink (POST /issues/)."""
+
+    model_config = ConfigDict(extra="forbid")
+
+    title: str = Field(..., min_length=1, max_length=500)
+    description: str = ""
+    target_repo: str = Field(..., min_length=1)
+    priority: Priority = "medium"
+    labels: List[str] = Field(default_factory=list)
+    due_in_days: Optional[int] = Field(default=None, ge=0)
+    source_type: SourceType
+    source_id: str = Field(..., min_length=1)
+    triggering_event_id: UUID
+    activity_definition_id: str = Field(..., min_length=1)
+
+
+class TaskIngestionResponse(BaseModel):
+    """Response returned to the emitter after a successful ingestion."""
+
+    issue_id: str
+    issue_url: Optional[str] = None
+    backend: BackendName
+
+
+class ErrorResponse(BaseModel):
+    """Uniform error envelope."""
+
+    error: str
+    detail: Optional[str] = None