Add project documentation and framework setup

- Initialize project documentation structure
- Add business case and stakeholder analysis
- Create project plan with four milestones
- Setup framework integration via git submodule
- Configure gitignore for project structure

docs/project-plan.md

@@ -0,0 +1,87 @@
+# Project Plan
+
+## Metadata
+| Key | Value |
+| --- | --- |
+| ID | PP-001 |
+| CrossReference | [MIL-001], [MIL-002], [MIL-003], [MIL-004], [SA-001], [BC-001] |
+
+## Version History
+| Date | Status | Author | Reviewer | Change | Commit |
+| --- | --- | --- | --- | --- | --- |
+| 2026-10-04 | Accepted | Jens Tirsvad Nielsen | S01 | Initial version | pending |
+
+---
+
+## Purpose
+
+Schedule the four phases that deliver the console calculator of Udemy's "100 Days of Code" day 10 assignment. This is a very small project, so the Business Case is short and the Stakeholder Analysis names only the Product Owner (S01); the plan, the milestones and their tasks are the whole planning trail. Each phase is one branch and one pull request, and each task is synced as an issue.
+
+## Planning Assumptions
+
+- Work starts 2026-10-05 and ends by 2026-10-08, one phase per day.
+- Owner and reviewer are the Product Owner, S01. The review is the Product Owner accepting the document.
+- Product Owner language: English.
+- Python 3.13 or newer, `venv`, `src/ tests/ docs/` layout, constants in `constants.py`, Doxygen comments.
+- `.env` holds a personal token used only for tooling; the project never reads or tests it.
+- Nothing is committed or pushed until the Product Owner asks.
+
+## Gateway Schedule
+
+| Gateway | Document | Window | Decision date | Owner | Stories | Main deliverable | Milestone |
+| --- | --- | --- | --- | --- | --- | --- | --- |
+| Project scaffold | [MIL-001] | 2026-10-05 | 2026-10-05 | S01 | | `pyproject.toml` | |
+| Calculator core | [MIL-002] | 2026-10-06 | 2026-10-06 | S01 | | `src/calc/constants.py` and `src/calc/operations.py` | |
+| Console interface | [MIL-003] | 2026-10-07 | 2026-10-07 | S01 | | `src/calc/calculator.py` and `src/calc/__main__.py` | |
+| Documentation and release | [MIL-004] | 2026-10-08 | 2026-10-08 | S01 | | `README.md` following the project template | |
+
+```plantuml
+@startgantt
+Project starts 2026-10-05
+[Project scaffold] starts 2026-10-05 and ends 2026-10-05
+[Project scaffold Go/No-Go] happens 2026-10-05
+[Calculator core] starts 2026-10-06 and ends 2026-10-06
+[Calculator core Go/No-Go] happens 2026-10-06
+[Console interface] starts 2026-10-07 and ends 2026-10-07
+[Console interface Go/No-Go] happens 2026-10-07
+[Documentation and release] starts 2026-10-08 and ends 2026-10-08
+[Documentation and release Go/No-Go] happens 2026-10-08
+@endgantt
+```
+
+## Scope Coverage
+
+| Scope item | Gateway |
+| --- | --- |
+| Project configuration, layout, `.gitignore`, Doxyfile, repository metadata | [MIL-001] |
+| Operations stored in a dictionary of functions, constants, tests | [MIL-002] |
+| Console interaction, ASCII art, continue with result, recursion | [MIL-003] |
+| README, Doxygen build, setup verification, AGPL licence | [MIL-004] |
+
+## Dependencies
+
+```
+MIL-001 -> MIL-002 -> MIL-003 -> MIL-004
+```
+
+A No-Go moves every later date by the time needed to fix the failed criterion.
+
+## Plan Risks
+
+| Risk | Impact | Mitigation |
+| --- | --- | --- |
+| Doxygen not installed on the machine | Docs check skipped | README names it as optional |
+| Recursion depth on very long sessions | Crash after many restarts | Documented limit; sessions are short |
+
+## Open Issues
+
+- Milestone links in the Gateway Schedule are filled in after `sync-project.sh --apply`.
+
+---
+
+[MIL-001]: ./milestones/mil-001-project-scaffold.md
+[MIL-002]: ./milestones/mil-002-calculator-core.md
+[MIL-003]: ./milestones/mil-003-console-interface.md
+[MIL-004]: ./milestones/mil-004-documentation-and-release.md
+[SA-001]: ./stakeholder-analysis.md
+[BC-001]: ./business-case.md