Docs(refactor[agents]): Route instead of restate

[tony]

AGENTS.md is read on every task, so anything in it that governs one class of change is context spent on every change that does not touch that class. This splits the policy in three and leaves the root file as a router.

Layout

• AGENTS.md — universal constraints, then one pointer per class of change, and nothing else.
• .github/WRITING.md — how we write: README prose, changelog, release notes, commit messages, API documentation, source comments, terminology, code blocks.
• .github/CONTRIBUTING.md — how we work: build, tests, required checks, pull request structure, review, releases, compatibility.

The root CONTRIBUTING.md is gone; .github/CONTRIBUTING.md replaces it. GitHub still links a contributing guide from that location, so the new-issue and new-pull-request pages are unaffected.

CLAUDE.md is a symlink to AGENTS.md, so it follows with no separate change.

The routes are written as conditions — "for changes to X, follow Y" rather than a bare "read Y" — so an agent re-evaluates per change instead of treating the pointer as a one-time startup instruction.

What the policy says

Two things in .github/WRITING.md are worth calling out:

• Commit format now has a single home. AGENTS.md on master mandated the Scope(type[detail]): prefix while CONTRIBUTING.md on master forbade prefixes outright. The repository's own history uses the prefix, so that is what is written down, and the contradiction is gone.
• The documentation rules are Javadoc rules, not generic ones: contract over mechanism, the first sentence as the summary the tool generates, and JSpecify carrying nullability because NullAway already enforces it in JSpecify mode.

.github/CONTRIBUTING.md absorbs the socket-hygiene rule in full. That was forty lines of test hygiene living in AGENTS.md — exactly the always-on context cost this refactor exists to remove. AGENTS.md keeps one line naming the socket roots, as a constraint plus a route rather than a restatement.

Nothing claims a gate that does not exist. No japicmp, no SBOM, no release machinery beyond what release.yml already does. japicmp in particular would contradict the README's own alpha terms, which promise no compatibility at all.

Repairs

Moving CONTRIBUTING.md under .github/ left two links resolving to nothing, and moving the socket rule out of AGENTS.md left six files citing a file that no longer explains what they cite it for. Both sets are repointed. docs/parity/test-map.md is deliberately untouched: its "per AGENTS.md" quotes the Python repository's file inside a row describing a Python test.

The alpha terms were stated three different ways — the README blockquote, the README Status section, and the CHANGELOG.md preamble. They now agree, in the wording every port shares: releases carry an -alpha prerelease tag, the API is not settled, any release may change or remove exported identifiers without a deprecation period, pin an exact version, not recommended for production.

The README title is libtmux for Java, matching the other ports.

Verification

./gradlew check passes. docs-tests re-ran rather than resolving from cache, so the Java snippets in the edited README were compiled and run against a real tmux. Every relative link in the touched files resolves on disk, and no <placeholder> remains in the tree.

.github/*.md is not covered by docs-tests, so the code blocks in the two policy files are gated by review alone.

Not in this branch

The heading sets in both policy files are shared with five sibling ports and are unchanged. Filling those ports is separate work.