Route publication contributions through public pull requests

[Brandon Miller (brandom-msft)]

Summary

• make a public same-repository pull request the normal contributor route for changes intended for publication
• require sample changes to use same-repository branches because the required trusted gate intentionally rejects fork pull requests
• state that required checks must pass and maintainers initiate normal contributor merges
• distinguish the retained manual, operator-only bridge for approved private-owned content: it is not a contributor route, publishes through a public pull request after required checks, and the sync App rebase-merges under the active public ruleset with no bypass actors
• remove private-staging, promotion, transition taxonomy, and automated handoff guidance

Scope

This PR changes only README.md and CONTRIBUTING.md. It does not change workflows, rulesets, CODEOWNERS, samples, notebooks, or repository settings.

Activation gate

Keep this PR in Draft until:

1. contributor wording is approved by the designated AI Platform Docs documentation owner;
2. the public trusted validation lane is restored after force-full sync commit 1a7f828fd60e6b0f14549612cf25e7bf2c906931 removed it;
3. P5.2 proves the constrained retained-bridge boundary: public mirror workflow 289478915 remains disabled; private sync workflow 233646160 remains manual and operator-only for approved private-owned content; the bridge opens a public pull request, waits for required checks, and the sync App rebase-merges under active public ruleset 6131793, whose bypass actor list is empty; the bridge is not documented or exposed as a contributor route.

At activation, merge this public guidance first through the normal human-triggered public merge path, then merge the coordinated private guidance in microsoft-foundry/foundry-samples-pr#895.

Tracking

• ADO 5502654
• ADO 5449716

Validation

• git diff --check
• final changed-file review limited to README.md and CONTRIBUTING.md
• normal pull-request checks, including required trusted, after restoration of the deleted validation lane

[Brandon Miller (brandom-msft)]

Sheri Gilley (@sdgilley) This is ready for AI Platform Docs owner review. The focused diff changes only README.md and CONTRIBUTING.md; P5.2 writer shutdown and public-only routing activation remain blocked on your explicit approval.