feat: accept documentation proposals from anyone

[carlok]

Closes the friction #290 documented. Twice now, @qazW12345 has tried to improve docs/CONTRIBUTION-DIRECTIONS.md and been refused by the receiver: an ordinary submission may change only LeanFrontier/ plus one claim, and the maintenance/ path is gated on OWNER, so a fork cannot propose prose at all. Both times a maintainer cherry-picked the commit instead (#200, #290).

The rule

A pull request whose every changed path is a Markdown file under docs/ is a documentation proposal: no claim, no Lean, no build.

• Decided from the changed paths, never from the branch name, keeping the project's own rule that a branch name grants nothing.
• docs/catalogue/ and docs/website/ are excluded. The first is generated from the corpus; the second is deployed to Pages, so a change there would publish under the project's name.
• Size limits still apply, deletions are still refused, and a mixed change (docs plus anything else) is an ordinary submission and fails as one.
• Nothing merges unattended: no mechanical check reads prose, so a person still reads it.

Workflow

The preflight now reports docs_only, and the three stages that build or run candidate code are gated on it:

stage	gated
Build trusted validation image	yes
Fetch pinned dependencies and Mathlib cache	yes
Restricted formal validation	yes

So a prose change costs one short job instead of a Docker build and a Mathlib fetch.

Evidence

• End to end with the real receiver: a docs-only diff against main → accepted: True, docs_only: True, changed_files: ['docs/CONTRIBUTION-DIRECTIONS.md'], no diagnostics, no Lean run.
• Tests (all failed before the change where they should): docs-only accepted without a claim; a new docs file accepted; docs mixed with Lean rejected; docs/website/ and docs/catalogue/ both rejected as path violations; a docs deletion rejected; and a workflow contract test pinning that each Lean stage carries the docs_only gate.

Full suite: 151 tests, 6 skipped.

CONTRACT.md §2 and CONTRIBUTING.md describe the new contribution type.

🤖 Generated with Claude Code