Add the Contributors Wall template (github-contributors-wall) #66

Part of #5. Thank your contributors with a wall of avatars.

Idea
The left side shows the repo name, a large "128 contributors" and a short thank-you line. The right side is a wall of up to 15 circular initials avatars that wrap. Each person's color is picked from a small palette based on their name, so it's the same every time. If the count is larger than the number of names shown, the last circle reads "+113".

Template details

• File: src/templates/github/contributors-wall.ts, id github-contributors-wall
• Fields: repoName (text), commonFields.description, contributorCount (text), contributors (textarea, comma-separated names), commonFields.bgColor, commonFields.accentColor, commonFields.textColor

Where to look

• Closest reference: src/templates/event/multi-speaker.ts (wrapping grid), src/templates/github/stars-banner.ts (big number)
• Deterministic pattern: src/templates/github/contribution-graph.ts

Acceptance criteria

• Initials are computed safely (one-word names, extra spaces)
• No Math.random(); colors derive from the name
• Re-exported in src/templates/github/index.ts and registered in the GitHub block of src/templates/registry.ts
• Meets the shared template checklist in Add new card-style OG image templates #5

One template per contributor at a time, so more people get a chance.

Want to work on this? Comment below and I'll assign it to you. Please take one issue at a time, branch off dev, and open your PR against dev with Closes #<this issue number>.