Add contribution guidelines and GitHub templates

Add CONTRIBUTING.md with instructions for proposing features, claiming
wishlist items, and marking items as completed. Include the list of
recognized categories and local development setup.

Add a pull request template with type checkboxes and a front matter
checklist. Add a structured GitHub issue form for proposing new kernel
feature ideas.

Signed-off-by: Christian Brauner <[email protected]>

Author: brauner

.github/ISSUE_TEMPLATE/feature-idea.yml

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+name: Feature Idea
+description: Propose a new kernel feature idea
+labels: ["wishlist"]
+body:
+  - type: input
+    id: title
+    attributes:
+      label: Feature title
+      description: A short, descriptive title for the feature.
+      placeholder: "e.g., Support detached mounts with pivot_root()"
+    validations:
+      required: true
+
+  - type: textarea
+    id: description
+    attributes:
+      label: Description
+      description: Describe the feature and how it would work.
+    validations:
+      required: true
+
+  - type: textarea
+    id: usecase
+    attributes:
+      label: Use-Case
+      description: Why is this feature valuable? What problem does it solve?
+    validations:
+      required: true
+
+  - type: dropdown
+    id: category
+    attributes:
+      label: Category
+      description: Which subsystem area does this relate to?
+      multiple: true
+      options:
+        - mounts
+        - pidfd
+        - namespaces
+        - filesystems
+        - sockets
+        - cgroups
+        - block-devices
+        - security
+        - io-uring
+        - processes
+        - other
+
+  - type: textarea
+    id: links
+    attributes:
+      label: Related links
+      description: Links to mailing list threads, patches, documentation, etc.

.github/PULL_REQUEST_TEMPLATE.md

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+## Feature
+
+<!-- Title of the feature being added, updated, or completed -->
+
+## Type
+
+<!-- Check one -->
+- [ ] New wishlist item
+- [ ] Moving item to in-progress
+- [ ] Marking item as completed
+- [ ] Other (describe below)
+
+## Checklist
+
+- [ ] Front matter is complete (`title`, `status`, `categories`)
+- [ ] Use-case section is included
+- [ ] Cc: Christian Brauner noted if implementing a feature from this list

CONTRIBUTING.md

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+# Contributing
+
+Thank you for your interest in the UAPI Group Kernel Feature Wishlist. This
+document explains how to propose new features, claim existing ones, or mark
+items as completed.
+
+## Proposing a new feature
+
+1. Create a new Markdown file in the appropriate section directory
+   under `website/content/`:
+   - `wishlist/` for new ideas
+   - `in-progress/` for features you are actively working on
+
+2. Use the following front matter template:
+
+   ```yaml
+   ---
+   title: "Short descriptive title"
+   weight: 10
+   status: wishlist
+   categories:
+     - mounts
+     - namespaces
+   ---
+   ```
+
+3. Include a clear description of the feature and a **Use-Case** section
+   explaining why this would be valuable.
+
+4. Open a pull request.
+
+## Claiming an item
+
+To indicate you are working on a wishlist item:
+
+1. Move the file from `website/content/wishlist/` to
+   `website/content/in-progress/`.
+2. Update the `status` field in the front matter to `in-progress`.
+3. Open a pull request with your GitHub handle or email address noted.
+
+## Marking an item as completed
+
+When a feature has been merged into the kernel:
+
+1. Move the file from its current location to `website/content/completed/`.
+2. Update the `status` field to `completed`.
+3. Add a `commit` field to the front matter with the commit SHA.
+4. Open a pull request.
+
+## Attribution
+
+**When implementing ideas on this list or ideas inspired by this list,
+please point that out explicitly and clearly in the associated patches
+and Cc `Christian Brauner <brauner (at) kernel (dot) org>`.**
+
+## Categories
+
+Use one or more of the following categories in front matter:
+
+- `mounts` — mount namespaces, pivot_root, move_mount, statmount
+- `pidfd` — pidfd xattrs, CLONE_PIDFD, SCM_PIDFD
+- `namespaces` — user namespaces, PID namespaces, mount namespaces
+- `filesystems` — nullfs, blobfs, overlayfs, tmpfs, binfmt_misc
+- `sockets` — AF_UNIX, SCM_RIGHTS, SCM_PIDFD
+- `cgroups` — device cgroups, coredump limits
+- `block-devices` — loop devices, dm-verity, diskseq
+- `security` — LSM hooks, user namespace restrictions
+- `io-uring` — io_uring extensions
+- `processes` — pidfd, clone3, waitid, prctl
+
+## Local development
+
+```sh
+git clone --recurse-submodules https://github.com/uapi-group/kernel-features.git
+cd kernel-features/website
+hugo server
+```