Add project plan, milestones and SQA framework; document actual capabilities

Adopt the SQA/QC framework as a submodule and plan the project before any
further code: Business Case, Stakeholder Analysis (S01-S05), Project Plan with
use case candidates and prioritisation, and milestones MIL-001..MIL-004
(24 tasks, synced to Gitea as milestones 18-21 and issues #1-#24).

README now separates implemented from planned capabilities, has an English
sequence diagram and points at the scoped workflow. The workflow file moves
out of the repository root and out of .gitea/workflows/; the scoped copy is
unchanged in behaviour here and still contains the known `exit 0` syntax
error, which MIL-001 fixes.

Refs #1
Refs #2
Refs #3
Refs #6

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

docs/artifact-registry.md

@@ -0,0 +1,38 @@
+# Artifact Registry
+
+This project's artifact state. Types, short names and `CrossReference
+Candidates` come from the framework catalog
+(`framework/registry/artifact-catalog.md`); this file only records where
+each document lives in *this* project and the next version to use.
+
+| Short Name | Artifact Type | Primary File | Next Available Version |
+| --- | --- | --- | --- |
+| BC | Business Case | docs/business-case.md | 002 |
+| SA | Stakeholder Analysis | docs/stakeholder-analysis.md | 002 |
+| PP | Project Plan | docs/project-plan.md | 002 |
+| MIL | Milestone / Gateway | docs/milestones/*.md | 005 |
+| RC | SQA Review Record | docs/sqa/reviews/rc-*.md | 001 |
+
+## Languages
+
+| Setting | Value |
+| --- | --- |
+| PO language | en (English, IT) |
+| High-level register | IT Executive English |
+| Technical register | IT Professional English |
+
+The PO language is English, so no PO-language translations are kept.
+
+| Artifact types | Register | Also kept as a PO-language file |
+| --- | --- | --- |
+| BC, KPI, PP, MIL | IT Executive English | No (PO language is English) |
+| SA, BMC, BPMN, UCD, US, UC, SSD, DM, RA, GOV, DICT | IT Professional English | No (PO language is English) |
+| OC, SD, DCD, ERD, ADR, TM, RC, QC, source code | IT Professional English | No |
+
+## Notes
+
+- "Next Available Version" is the zero-padded (3-digit) version to use the
+  *next* time a new document of that type is created. Increment it only when
+  a brand-new document is created, not when an existing document's
+  `## Version History` gets a row.
+- `ADR` uses 4 digits (`0001`); `RC` is sequential across all artifact types.