Agentic software factory Part 9: Contribution model: outside PRs, CONTRIBUTING.md, PR template, issue forms

[cliffhall]

Part of the agentic software factory tracker #4858, Wave 3. Proposed as S8 in docs/agent-guidance-inception.md (PR #4861) §9, from #4859.

Depends on

After Part 2 (AGENTS.md). Needs a maintainer decision.

Scope

• A maintainer decision: keep accepting outside PRs, accept only bug fixes with a linked issue, or go issues-only like the Inspector. See §10 of the doc.
• Then make these match the decision:
  • CONTRIBUTING.md
  • .github/pull_request_template.md
  • new issue forms in .github/ISSUE_TEMPLATE/: bug and feature forms with a server dropdown and a spec-era/client field; config.yml routing security reports to a private advisory and new servers to the Registry

Acceptance criteria

• A maintainer records the decision on this issue.
• The docs and templates match it.
• The forms validate against GitHub's issue-forms schema. They only go live after the next milestone merge to main, since GitHub serves them from the default branch.

Branch

Targets v2/main. Closes #N won't auto-close an issue on a non-default branch, so close this one by hand on merge and move the card to Done.