docs: add AGENTS.md stub

AGENTS.md

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+# machinevision-toolbox-python — Agent Instructions
+
+Part of the RVC ecosystem. **Read [rvc-ecosystem/AGENTS.md](https://github.com/petercorke/rvc-ecosystem/blob/main/AGENTS.md) first** — it defines shared conventions: repo ownership, math invariants, dependency boundaries, git/PR workflow, code standards, tech-debt tracking. This file only adds what's specific to this repo.
+
+| | |
+|---|---|
+| PyPI package | `machinevision-toolbox-python` |
+| Nickname | MVTB |
+| Owner | Peter Corke (`petercorke`) |
+| Default branch | `main` |
+| Contribution model | Branch → PR; direct push to `main` at Peter's discretion |
+
+## Notes specific to this repo
+
+- Pure Python — no compiled extensions, builds with Hatch (`hatchling`) directly.
+- Depends on `spatialmath`, `pgraph-python`, `ansitable`, plus the third-party `opencv-python`/
+  `opencv-contrib-python` and `open3d`.
+- `open3d` is consistently a Python version or two behind — never assume it supports the
+  latest Python release; check compatibility before bumping this repo's Python floor/ceiling.
+- OpenCV 5.x support was just added alongside the existing 4.x support (a major version jump,
+  not a minor bump). This is fresh work — aim to keep both working, but treat 4.x/5.x
+  compatibility friction as expected teething problems to fix, not a regression.
+- Tech-debt tracked as GitHub Issues labelled `tech-debt` (migrated 2026-08-02) — this repo
+  was first to migrate off `tech-debt.md`.
+- This repo's `.github/pull_request_template.md` and `.github/ISSUE_TEMPLATE/*.yml` are the
+  canonical source other repos' templates were copied from — check here first if updating
+  template content ecosystem-wide.
+- Codacy is wired in (badge + automated PR comments); grade A is the aspiration. Real backlog
+  exists, tracked via `tech-debt`-labelled issues (including a "mypy not in CI" entry) — see
+  ecosystem `AGENTS.md` §5 for how that target is framed.