Docs: add fork-specific PR guidance to main [AI-assisted]

[artemgetmann]

Review Fast Path

• User path fixed: fork PR authoring on main now uses the same fork-specific review guide as consumer instead of defaulting to the noisy upstream contribution flow.
• Proof: AGENTS.md on main now routes fork PRs targeting main or codex/consumer-openclaw-project to FORK_CONTRIBUTING.md, and the new guide defines the exact fast-review block plus concise PR structure.
• Shared-state footgun removed: agents no longer have to guess whether founder-main fork PRs should use upstream CONTRIBUTING.md.
• Still hurts: this is guidance only; people can still ignore it until the workflow sticks.

Why This Matters

• The fork has two active lines: founder main and consumer integration.
• Both benefit from the same fast-review PR shape.
• Keeping the rule only on consumer would leave main falling back to the slower upstream-style PR ritual.

Scope Boundary

• Added FORK_CONTRIBUTING.md on main.
• Updated AGENTS.md PR guidance routing on main.
• Did not change runtime or product behavior.
• Did not change upstream PR workflow.

Verification

• git diff --check
• reviewed the exact AGENTS.md routing rule
• reviewed the new fork guide contents end-to-end

AI Assistance

• AI-assisted
• Testing degree: targeted