* docs: name the five supported targets and drop Plan 9 as a review gate
Plan 9 has never compiled and is not a release or CI target. Reviews still
asked for plan9 shims. Document the five release platforms as the supported
set so unsupported GOOS is not a review criterion.
FixesGitlawb/zero#817
* docs: exclude unsupported GOOS/GOARCH from review criteria
CONTRIBUTING.md names five exact release combinations. Align AGENTS.md and
.coderabbit.yaml so other GOOS/GOARCH pairs are not a review criterion.
Unsupported targets such as plan9 remain allowed as fixtures.
* docs: retain Android support in target policy
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Co-authored-by: cairn-intern <[email protected]>
Documents expectations for keeping contribution flow sustainable during the
stabilization phase: one issue per bug (no bundled multi-finding audits), keep
only a few items open at a time, and hold AI-assisted contributions to the same
per-item verification bar. Complements the existing issue-approved gate, which
already covers PR-to-issue linkage and scope; the gap was submission volume.
