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/pr-submit/SKILL.md

@@ -0,0 +1,28 @@
+---
+name: pr-submit
+description: Create and submit a GitHub PR from the current branch
+---
+
+Submit the current changes as a GitHub pull request.
+
+## Instructions
+
+1. Check the current state of the repository:
+   - Run `git status` to see staged, unstaged, and untracked changes
+   - Run `git diff` to see current changes
+   - Run `git log --oneline -10` to see recent commits
+
+2. If there are uncommitted changes relevant to the PR:
+   - Ask the user if they want a specific prefix for the branch name (e.g., `alice/`, `fix/`, `feat/`)
+   - Create a new branch based on the current branch
+   - Commit the changes using multiple commits if the changes are unrelated
+
+3. Push the branch and create the PR:
+   - Push with `-u` flag to set upstream tracking
+   - Create the PR using `gh pr create`
+
+4. After the PR is created:
+   - Run `/changelog <pr_number>` to generate changelog files, then commit and push them
+   - Run `/pr-description <pr_number>` to update the PR description
+
+5. Return the PR URL to the user.