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>

CHANGELOG.md

@@ -85,7 +85,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Enhanced agent integration documentation
 
 ### Fixed
-- ID mapping bugs in issue-facade
+- ID mapping bugs in issue-core
 - Sync metadata handling
 - Backend initialization edge cases
 
@@ -103,7 +103,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [0.1.0] - 2024-10-06
 
 ### Added
-- Initial extraction of issue-facade as standalone capability
+- Initial extraction of issue-core as standalone capability
 - Core CRUD operations (create, read, update, delete issues)
 - Gitea backend implementation (production-ready)
 - Local SQLite backend (offline capability)
@@ -173,6 +173,6 @@ To submit feedback, see [feedback/README.md](feedback/README.md).
 
 ## Links
 
-- [Repository](http://92.205.130.254:32166/coulomb/issue-facade)
-- [Issues](http://92.205.130.254:32166/coulomb/issue-facade/issues)
+- [Repository](http://92.205.130.254:32166/coulomb/issue-core)
+- [Issues](http://92.205.130.254:32166/coulomb/issue-core/issues)
 - [MarkiTect Project](https://github.com/markitect)