docs(contribution-model): record that outside PR creation is turned off

[cliffhall]

Closes #4966

Description

Updates "New outside PRs after the sweep" in docs/contribution-model.md. It said nothing stopped new outside PRs and listed automatic closing as an open maintainer decision. On 2026-10-04, once the policy reached main (#4939), pull request creation was restricted in the repository settings.

The section now records the setting and what triage still handles: the open backlog, and PRs from accounts that keep access but are not maintainers (write access alone does not make someone one, so the policy still covers them). The automatic-closing bullet now says the setting settled it, for accounts without access. The readme-pr-check.yml bullet is unchanged.

Server Details

• Server: none (repository-wide)
• Changes to: docs

Motivation and Context

See #4966.

How Has This Been Tested?

No client-observable surface; targeted probe: git grep -n -i -E "nothing in this change stops|automatic closing|repository setting|new outside PRs" over AGENTS.md, CONTRIBUTING.md, docs, .claude and .github finds no other text that assumes outside PRs can still be opened by anyone. Markdown is not a Prettier target here, and no code, package, workflow or skill is touched, so npm run local:gate was not run.

Breaking Changes

None.

Types of changes

• Bug fix
• New feature
• Breaking change
• Documentation update

Checklist

• I have read the MCP Protocol Documentation (not applicable: no protocol surface)
• My changes follow MCP security best practices (not applicable: docs only)
• I have updated the server's README accordingly (not applicable: no server change)
• I have added a changeset (not applicable: nothing published changes)
• I have tested this with an LLM client (not applicable: see the probe above)
• My code follows the repository's style guidelines (not applicable: no code)
• New and existing tests pass locally (not applicable: no code)
• I have added appropriate error handling (not applicable: no code)
• I have documented all environment variables and configuration options (not applicable: none added)

🤖 Generated with Claude Code

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Latest commit: d39af49

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[Copilot]

Copilot review overview

🟢 Approval recommended

The documentation matches the linked issue and existing contribution policy, with no unresolved issues.

Review effort: Balanced
Findings: None

What changed in this PR

Updates contribution-policy documentation to reflect the restriction on outside pull request creation.

Changes:

• Records the repository setting enabled on 2026-10-04.
• Clarifies that triage still handles the backlog and PRs from non-maintainers who retain access.

File	Description
docs/​contribution-model.md	Records the creation restriction and remaining triage scope.

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[cliffhall]

Copilot round 1: approval recommended, no findings. Clean round; review loop done.