docs: add OSS Contributor Accelerator template example

[Deeven-Seru]

Fixes #5688

Summary

Adds a new example template, OSS Contributor Accelerator, to help contributors move from repository context to a shippable contribution plan.

What’s included

• New template folder: examples/templates/oss_contributor_accelerator
  • agent.py
  • nodes/__init__.py
  • config.py
  • __main__.py
  • agent.json
  • mcp_servers.json
  • README.md
  • DEMO.md
• Template index update in examples/templates/README.md

Flow

intake -> issue-scout -> selection -> contribution-pack

• intake: collect repo + contributor profile
• issue-scout: use GitHub tools to shortlist/rank 8 high-leverage issues
• selection: human selects 1–3 issues
• contribution-pack: generate contribution_brief.md with implementation plan, tests, PR draft text, and maintainer update draft

Validation performed

• uv sync
• uv run ruff check examples/templates/oss_contributor_accelerator
• uv run python -m examples.templates.oss_contributor_accelerator validate