docs: convert all architecture .adoc sections to markdown, prettier-format all .md

- Converted 13 individual .adoc section files to .md via
  asciidoctor→docbook5→pandoc(gfm) + post-processing, placed alongside
  the .adoc sources in Docs/Architecture/
- Rebuilt architecture.md from the individual section .md files
- Ran prettier --write on all .md in the repo (23 files changed):
  normalised table column alignment, trailing whitespace, line breaks
- Added .agents/ to .prettierignore (vendored third-party content)
- All .md now passes prettier --check

Docs/Architecture/02_architecture_constraints.md

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+# Architecture Constraints
+
+## Technical Constraints
+
+| Constraint | Explanation |
+| --- | --- |
+| Engine: Unreal 5.8.4 | Pinned; no bundled Linux clang — uses system clang 23.1.4. Game targets use `BuildSettingsVersion.V7`. |
+| Platform: Linux first | Developed and tested on Bazzite Linux (Fedora 45 base). Windows builds ship but are secondary. |
+| GPU: RTX 5070 Ti 16 GB | Dev workstation target. Vulkan RHI; open kernel module driver 615.78.08. |
+| No static lighting | `r.AllowStaticLighting=False`. The demo is a fixed daytime scene lit entirely dynamically to avoid "LIGHTING NEEDS TO BE REBUILT" warnings. |
+| Blender→UE via FBX | Flat diffuse colours do not survive FBX→Unreal import. All materials are authored in UE. |
+| Terrain collision | UE 5.8 exposes `CollisionTraceFlag` (not `bUseComplexAsSimpleCollision`); Python cannot set it. Must use C++ helper `UGigLineAssetTools::SetComplexCollisionAsSimple`. |
+| Coordinate convention | South = +Y, north = −Y (ground truth = street mesh orientation). 1 m in Blender = 100 cm in Unreal. |
+
+## Organisational Constraints
+
+| Constraint | Explanation |
+| --- | --- |
+| Squash-merge to `main` | Every PR is squash-merged for a linear history. No direct commits to `main` except integration merges. |
+| Visual review required | Any change affecting appearance must produce screenshots for human approval before being marked done. |
+| Asset provenance | Every committed asset must have verified licence and redistribution permission recorded in `THIRD_PARTY_NOTICES.md`. |
+| Reference photography | Local only, gitignored, never committed. Used for façade work under fair-use / internal-development rationale. |
+| LFS for binaries | `.uasset`, `.umap`, `.blend` tracked with Git LFS; `Binaries/`, `Intermediate/`, `Saved/` are gitignored. |
+
+## Conventions
+
+| Convention | Explanation |
+| --- | --- |
+| Semantic versioning | `MAJOR.MINOR.PATCH`. `0.x` documents breaking changes. `1.0.0` = agreed game scope. |
+| Task tracking in `TODO.md` | Single central task list. `- [ ]` open, `- [-]` or `- [~]` in progress, `- [x]` done, `- [A]` active, `- [D]` deferred. |
+| Headless Blender | All Blender work driven by `bpy` scripts in `Tools/`, not MCP. Blender skills are method guides only. |
+| Named export presets | `ue_metric_v1` for static meshes, `ue_character_v1` for skeletal. Every exporter calls the metre→centimetre conversion. |