Restore dev skills and add marketplace for maintainer workflows

Brings back the 6 development workflow skills (changelog, cleanup,
code-review, docstring, pr-description, pr-submit) that were moved
to pipecat-ai/skills, and adds a .claude-plugin/marketplace.json so
other pipecat-ai repos can install them. Updates README contributing
section with installation instructions.

.claude/skills/changelog/SKILL.md

@@ -0,0 +1,47 @@
+---
+name: changelog
+description: Create changelog files for important commits in a PR
+---
+
+Create changelog files for the important commits in this PR. The PR number is provided as an argument.
+
+## Instructions
+
+1. Skip changelog for: documentation-only, internal refactoring, test-only, CI changes.
+
+2. First, check what commits are on the current branch compared to main:
+   ```
+   git log main..HEAD --oneline
+   ```
+
+3. For each significant change, create a changelog file in the `changelog/` folder using the format:
+   Allowed types: `added`, `changed`, `deprecated`, `removed`, `fixed`, `security`, `performance`, `other`
+   - `{PR_NUMBER}.added.md` - for new features
+   - `{PR_NUMBER}.added.2.md`, `{PR_NUMBER}.added.3.md` - for additional entries of the same type
+   - `{PR_NUMBER}.changed.md` - for changes to existing functionality
+   - `{PR_NUMBER}.fixed.md` - for bug fixes
+   - `{PR_NUMBER}.deprecated.md` - for deprecations
+   - `{PR_NUMBER}.removed.md` - for removed features
+   - `{PR_NUMBER}.security.md` - for security fixes
+   - `{PR_NUMBER}.performance.md` - for performance improvements
+   - `{PR_NUMBER}.other.md` - for other changes
+
+4. Each changelog file should at least contain a main single line starting with `- ` followed by a clear description of the change. No line wrapping.
+
+5. If the change is complicated, changelog files can have indented lines after the main line with additional details or code samples.
+
+6. Use ⚠️ emoji prefix for breaking changes.
+
+## Example
+
+For PR #3519 with a new feature and a bug fix:
+
+`changelog/3519.added.md`:
+```
+- Added `SomeNewFeature` for doing something useful.
+```
+
+`changelog/3519.fixed.md`:
+```
+- Fixed an issue where something was not working correctly.
+```