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>

.capability/agent-context.md

@@ -1,4 +1,4 @@
-# Issue Facade - Agent Integration Context
+# Issue Core - Agent Integration Context
 
 **🤖 For Coding Agents: Read this to understand how to use issue tracking in this project.**
 
@@ -25,15 +25,15 @@
 # Verify installation
 issue --version
 # or
-python -c "from issue_tracker.backends.gitea import GiteaBackend; print('OK')"
+python -c "from issue_core.backends.gitea import GiteaBackend; print('OK')"
 ```
 
 ### Basic Usage (Python)
 
 ```python
-from issue_tracker.backends.gitea import GiteaBackend
-from issue_tracker.core.models import Issue, Label, IssueState, User, Comment
-from issue_tracker.core.interfaces import IssueFilter
+from issue_core.backends.gitea import GiteaBackend
+from issue_core.core.models import Issue, Label, IssueState, User, Comment
+from issue_core.core.interfaces import IssueFilter
 from datetime import datetime, timezone
 import os
 
@@ -228,7 +228,7 @@ if not verify_issue_backend():
 ## Error Handling
 
 ```python
-from issue_tracker.backends.gitea.backend import GiteaAPIError
+from issue_core.backends.gitea.backend import GiteaAPIError
 
 try:
     issue = backend.get_issue_by_number(42)
@@ -317,7 +317,7 @@ If you're unsure whether to use this capability for something:
 - **NO** → You can use other methods
 
 **Example:**
-- "Create an issue for the bug I found" → **Use issue-facade**
-- "Read the project README" → Don't need issue-facade
-- "Check if issue #42 exists" → **Use issue-facade**
-- "Clone the repository" → Don't need issue-facade
+- "Create an issue for the bug I found" → **Use issue-core**
+- "Read the project README" → Don't need issue-core
+- "Check if issue #42 exists" → **Use issue-core**
+- "Clone the repository" → Don't need issue-core