feat: support custom additional information in PR welcome message

[rnetser]

Problem

The PR welcome message only displays server-managed content (commands, labels, merge requirements). There's no way for repository owners to add project-specific information — deployment checklists, review guidelines, team contacts, or repo-specific notes.

Solution

Add a welcome-extra-info feature that renders user-provided content as an "Additional Information" section at the end of the PR welcome message. Content is injected as-is (markdown).

Priority Chain (first match wins)

Priority	Source	Location
1 (highest)	.github-webhook-server-welcome-message.md	In the repo (default branch)
2	.github-webhook-server.yaml → welcome-extra-info	In the repo (default branch)
3	config.yaml → repositories.X.welcome-extra-info	Server config, per-repo
4 (lowest)	config.yaml → welcome-extra-info	Server config, global

Config

# Global (config.yaml)
welcome-extra-info: "Please review the contribution guide before merging."

# Per-repo (config.yaml or .github-webhook-server.yaml)
welcome-extra-info: |
  **Important:** All PRs must include unit tests and documentation updates.

File-based (.github-webhook-server-welcome-message.md)

A markdown file in the repo root (read from the default branch). Supports rich markdown — tables, checklists, links. Max 10KB.

Rendering

Section appears at the end of the welcome message:

... (existing sections) ...

#### Additional Information
<content from file or config, injected as-is>

For more information...

Done

• Schema: welcome-extra-info (global + per-repo string field)
• Load .github-webhook-server-welcome-message.md from repo default branch
• Priority chain: file → repo YAML → per-repo config → global config
• Render as "Additional Information" section at end of welcome message
• /regenerate-welcome includes the section
• Remove all custom-commands code from the branch
• Tests: config loading, file loading, priority, rendering, empty/missing cases
• Schema validation tests
• Update examples/config.yaml

Design Decisions (Updated)

Section Header Format

The section uses ### 📌 Additional Information (h3 with pin emoji), matching the heading level used by all other welcome message sections (Tips, Merge Requirements, Review Process, etc.). This is a deliberate design choice — not the originally specced #### Additional Information — made during implementation to maintain visual consistency.

Content Processing

File content is .strip()-ed before injection. This is standard file-reading behavior — leading/trailing whitespace in files is typically unintentional (trailing newlines from editors, etc.). GitHub's markdown renderer ignores leading/trailing whitespace anyway, so .strip() has no visual effect while keeping the stored value clean.