docs: add PR and issue templates

.github/ISSUE_TEMPLATE/bug_report.yml

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+name: Bug report
+description: Report unexpected behavior or a crash
+title: "[Bug]: "
+labels: ["bug"]
+body:
+  - type: input
+    id: version
+    attributes:
+      label: MVTB version
+      description: Output of `pip show machinevision-toolbox-python`
+      placeholder: "e.g. 1.2.3"
+    validations:
+      required: true
+  - type: input
+    id: python-version
+    attributes:
+      label: Python version
+      placeholder: "e.g. 3.12.4"
+    validations:
+      required: true
+  - type: dropdown
+    id: os
+    attributes:
+      label: Operating system
+      options: [Linux, macOS, Windows, Other]
+    validations:
+      required: true
+  - type: textarea
+    id: expected
+    attributes:
+      label: Expected behavior
+    validations:
+      required: true
+  - type: textarea
+    id: actual
+    attributes:
+      label: Actual behavior
+    validations:
+      required: true
+  - type: textarea
+    id: repro
+    attributes:
+      label: Minimal reproducible example
+      description: A short, self-contained code snippet that reproduces the issue
+      render: python
+    validations:
+      required: true

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+contact_links:
+  - name: Ask a question
+    url: https://github.com/petercorke/machinevision-toolbox-python/discussions
+    about: For general questions, ideas, and discussion — not a good fit for the two forms above.

.github/ISSUE_TEMPLATE/feature_request.yml

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+name: Feature request
+description: Suggest a new feature or enhancement
+title: "[Feature]: "
+labels: ["enhancement"]
+body:
+  - type: textarea
+    id: problem
+    attributes:
+      label: What problem are you trying to solve?
+    validations:
+      required: true
+  - type: textarea
+    id: proposal
+    attributes:
+      label: Proposed solution
+      description: >-
+        What would the MVTB function/method look like? Include its
+        signature — parameter names and types, and the return
+        value(s) and type(s).
+      placeholder: |
+        def new_method(self, threshold: float, mode: Literal["a", "b"] = "a") -> NDArray:
+            ...
+    validations:
+      required: false
+  - type: textarea
+    id: alternatives
+    attributes:
+      label: Alternatives considered
+      description: >-
+        Any existing MVTB function that's close, a workaround you're
+        currently using, or another API shape you considered and why
+        it didn't fit. Leave blank if none.
+      placeholder: |
+        e.g. "Currently chaining img.threshold(...) then manual masking —
+        works but takes 3 extra steps and doesn't handle the RGBA case."
+    validations:
+      required: false

.github/pull_request_template.md

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+Thanks for contributing to MVTB!
+
+## Summary
+
+<!-- What does this PR do, and why? -->
+
+## Related issue
+
+<!-- Fixes #123 / Closes #123 — if applicable -->
+
+## Checklist
+
+- [ ] PR title follows [Conventional Commits](https://www.conventionalcommits.org/) (`type: description`) — checked automatically, see the "Check PR title" status
+- [ ] Tests pass locally (`pytest`)
+- [ ] Added/updated tests for this change, if applicable
+- [ ] New/changed code is type-hinted with modern syntax (`X | Y`, `list[X]`, not `Union`/`Optional`/`List`)
+- [ ] Docstrings updated (reST style: `:param:`, `:returns:`; type hints in the signature cover types now, `:type:`/`:rtype:` are rarely needed)
+
+> **Automated bot comments to expect:** Codacy will comment with style/coverage findings — most of it is pre-existing backlog (see `tech-debt.md`), not something your PR introduced, so don't be alarmed. If this PR adds or bumps a dependency, Dependabot may separately comment flagging a known vulnerability — if so, use a newer patched version if one exists, or just note it in this PR and a maintainer will decide; you don't need to solve it yourself.
+
+<!-- Target branch is `main` — release-please handles versioning automatically. -->
+