Add CONTRIBUTING.md, issue templates, and PR template (#125)

.github/ISSUE_TEMPLATE/bug_report.yml

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+name: Bug Report
+description: File a bug report
+labels: [bug]
+body:
+  - type: textarea
+    id: description
+    attributes:
+      label: Description
+    validations:
+      required: true
+  - type: textarea
+    id: steps
+    attributes:
+      label: Steps to Reproduce
+      render: julia
+    validations:
+      required: true
+  - type: input
+    id: version
+    attributes:
+      label: Version
+    validations:
+      required: true

.github/ISSUE_TEMPLATE/config.yml

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+blank_issues_enabled: false
+contact_links:
+  - name: Discussions
+    url: https://github.com/harmoniqs/DirectTrajOpt.jl/discussions
+    about: Questions and discussion.

.github/ISSUE_TEMPLATE/feature_request.yml

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+name: Feature Request
+description: Suggest an idea
+labels: [enhancement]
+body:
+  - type: textarea
+    id: motivation
+    attributes:
+      label: Motivation
+    validations:
+      required: true
+  - type: textarea
+    id: proposal
+    attributes:
+      label: Proposed Solution

.github/pull_request_template.md

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+## Related Issue
+Closes #<issue-number>
+
+## Description
+
+## Type of Change
+- [ ] Bug fix
+- [ ] New feature
+- [ ] Documentation
+- [ ] Infrastructure
+
+## Verification
+- [ ] `julia runtests.jl` exits 0

CONTRIBUTING.md

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+# Contributing to DirectTrajOpt.jl
+
+Welcome! DirectTrajOpt.jl provides the low-level optimization infrastructure
+for [Piccolo.jl](https://github.com/harmoniqs/Piccolo.jl) — trajectory
+parametrizations, integrators, and solver backends. Licensed under MIT.
+
+Every code change needs a GitHub issue and a PR. Create one first if it does
+not exist.
+
+## Dev Setup
+
+```bash
+git clone git@github.com:harmoniqs/DirectTrajOpt.jl.git && cd DirectTrajOpt.jl
+julia --project -e 'using Pkg; Pkg.instantiate()'
+```
+
+## Running Tests
+
+```bash
+julia --project test/runtests.jl
+```
+
+## Submitting a PR
+
+1. Create an issue describing the change.
+2. Branch from `main`.
+3. Commit focused changes with descriptive messages.
+4. Run the test suite.
+5. Open a draft PR with `Closes #<N>` at the first commit.
+6. Mark ready when green.
+
+## Getting Help
+
+Use [GitHub Discussions](https://github.com/harmoniqs/DirectTrajOpt.jl/discussions) for questions.