feat: WP-0006 — packaging & distribution infrastructure

CI matrix (Python 3.11/3.12) with pip cache and coverage job. PyPI publish
workflow (OIDC trusted publishing, triggered on v*.*.* tags). Docker image
for REST service with non-root user + ghcr.io push workflow. markidocx
--version flag. Diagram optional extras (diagram-mermaid/graphviz/plantuml)
and readme/urls in pyproject.toml. CHANGELOG.md (Keep a Changelog format)
with retrospective v0.1.0 entry. docs/release-process.md with executable
checklist. All 272 tests pass; ruff and mypy clean; twine check PASSED.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

workplans/MRKD-WP-0006-packaging-distribution.md

@@ -3,7 +3,7 @@ id: MRKD-WP-0006
 type: workplan
 domain: markitect
 repo: marki-docx
-status: active
+status: done
 state_hub_workstream_id: 7e255145-8d18-4f22-b1ca-31f02944b890
 created: 2026-03-16
 updated: 2026-03-16
@@ -25,7 +25,7 @@ release artefact production.
 
 ```task
 id: MRKD-WP-0006-T01
-status: todo
+status: done
 priority: high
 state_hub_task_id: cfca0094-b5ae-45ec-80c9-e7705f37bd12
 ```
@@ -54,7 +54,7 @@ Deliverable: `.github/workflows/ci.yml` present; CI passes on a clean push to `m
 
 ```task
 id: MRKD-WP-0006-T02
-status: todo
+status: done
 priority: high
 state_hub_task_id: 44f60455-b144-48e3-93a0-74869018c2ea
 ```
@@ -84,7 +84,7 @@ returns the correct version string.
 
 ```task
 id: MRKD-WP-0006-T03
-status: todo
+status: done
 priority: medium
 state_hub_task_id: 05ff77b0-6347-4c96-9fcd-c2b2baaf0fce
 ```
@@ -121,7 +121,7 @@ Deliverable: `docker build . && docker run -p 8080:8080 <image>` starts the REST
 
 ```task
 id: MRKD-WP-0006-T04
-status: todo
+status: done
 priority: medium
 state_hub_task_id: 177e4861-d153-4b1d-85d4-a272da14bfe5
 ```