docs: P1 backlog — converge dev-stack duplication, unify gate and SBOM-upload naming

[haksungjang]

Summary

The three P1 items from the 2026-07 documentation benchmark review (drift-prone duplication and split vocabulary).

1. Dev-stack boot steps: last duplication converged

contributor-guide/getting-started.md re-stated the dev-stack bring-up independently — and had already drifted: it skipped the env-file and schema-migration steps, so following it verbatim left the health-gated worker blocking up. It now delegates to [Quickstart steps 1–3] and keeps only the contributor-specific extras (logs, ports, host-local backend/frontend variants). This was the last of the three duplicated boot sequences (quickstart was made canonical and docker-compose delegated in #505).

2. Terminology: "policy gate" → "build gate"

The same mechanism was named differently per page (analysis-types/faq said policy gate; github-actions/scans/triage said build gate). Unified to build gate across current docs. Deliberately untouched:

• the glossary alias and the projects.md reconciliation sentence ("also called the policy gate") — kept so the old name still resolves;
• release notes (historical records);
• code identifiers (POST …/policy-gate, policy_gate_failed).

Renamed headings carry their old anchors explicitly ({#gate-detail}, {#build--policy-gate}, KO mirror), so no inbound link breaks.

3. Naming: one feature name for SBOM upload

The upload feature went by four names ("Received SBOMs (uploaded)", "Upload an SBOM", "uploaded SBOM", API sbom-ingest). Section titles and link texts now say SBOM upload (KO: SBOM 업로드); the how-to page keeps its task-style title ("Upload an SBOM"), descriptive uses of "an uploaded SBOM" as a noun phrase remain, and the #received-sboms-uploaded anchor is preserved.

Verification

• Docusaurus EN+KO build green — 0 broken links/anchors (all renamed headings keep explicit anchors)
• docs-uat extract green — 201 steps, ids/KO parity unchanged
• ko-style lint S1/S2/S3 all 0

Docs-only change — CI checks will skip; needs an --admin merge per repo convention.