docs(contribution-model): drop the transitional blank-issue wording

[cliffhall]

Closes #4962

Description

Rewrites the paragraph under "Where the policy is stated" in docs/contribution-model.md. It said the forms go live "at the first milestone merge that contains them" and that "until then the chooser offers a blank issue". The forms reach main with #4939, and config.yml disables blank issues, so that wording goes stale. It now states the steady-state rule, as CONTRIBUTING.md (#4959) and the issue-form headers do: a change to the forms or config.yml goes live at the next milestone merge, not when it merges into v2/main.

The close comments in issue-triage keep linking to CONTRIBUTING.md on v2/main, so the sentence about them stays, reworded.

Server Details

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

Motivation and Context

Same staleness as #4940, in a file outside #4939's five, so it would otherwise reach main at v2.0.0 unchanged. See #4962.

How Has This Been Tested?

No client-observable surface; targeted probe: git grep -n -i "blank issue\|until then" over docs, .claude, AGENTS.md, CONTRIBUTING.md and RELEASING.md now finds only config.yml's "disables blank issues" table entry, and no other transitional wording about the forms. Markdown is not a Prettier target in this repo, 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)

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Latest commit: 32571ef

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[Copilot]

Copilot review overview

🟢 Approval recommended

The documentation-only change matches the linked issue and repository guidance, with no unresolved findings.

Review effort: Balanced
Findings: None

What changed in this PR

Updates contribution-policy documentation to describe the steady-state issue-form rollout.

Changes:

• Replaces transitional blank-issue wording with the next-milestone-merge rule.
• Preserves the explanation for policy links targeting v2/main.

File	Description
docs/​contribution-model.md	Clarifies when form updates go live and why policy links target v2/main.

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[cliffhall]

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