chore: add project standards (AGENTS.md, CI, devlog, e2e scaffold)

[ranxianglei]

What

Add project standards to billion-context-opencode, bringing it to the same contributing standard as opencode-acp (adapted for this MIT monorepo with a single published package).

Files

File	Purpose
AGENTS.md (~436 lines)	MIT monorepo dev spec. Documents the dual-shape mechanism (Object.assign(v1Fn, {id, setup}) + why Plugin.define identity makes one package serve V1+V2), @bili/core private workspace, call↔result pairing invariant, splice-in-place reassembly, per-session lock contract, gh-guard note, git-safety table, and the absolute PR-merge prohibition (human-only).
.github/workflows/ci.yml	Matrix Node 22/24 → npm ci → typecheck → build → test. Job named build so branch protection can require it.
.github/workflows/pr-checks.yml	Runs scripts/ci/check-pr.sh (branch regex, devlog, changelog-on-bump). Job named pr-validation.
.github/workflows/release.yml	On push to master: detects release-branch merge (YYYY-MM-DD_release-v*, standard + squash patterns) → tag v{VERSION} → npm publish --workspace billion-context-opencode → GitHub Release. Prerelease (- suffix) → npm dev tag. workflow_dispatch(force) override. Publishes only the public package.
scripts/ci/check-pr.sh	Branch name ^\d{4}-\d{2}-\d{2}_[a-z0-9.-]+$, devlog REQ.md+WORKLOG.md existence, changelog entry when packages/billion-context-opencode/package.json version bumps.
devlog/{README,REQ.template,WORKLOG.template,DESIGN.template}.md	Naming convention + lighter templates adapted from opencode-acp.
devlog/2026-08-13_project-standards/{REQ,WORKLOG}.md	This task's devlog entry (so check-pr passes).
scripts/e2e/{README.md,run-e2e.sh}	E2e scaffold only — README documents the planned Docker + fake-LLM + state-asserting harness (deferred); run-e2e.sh is a stub. Full e2e is a follow-up.
README.md	Append ## Changelog with ### v0.1.0 baseline entry (existing content preserved).

Adaptations vs opencode-acp

• License: MIT (not AGPL-3.0).
• Monorepo: npm workspaces; CI runs npm run typecheck --workspaces, build/test target the billion-context-opencode workspace.
• Single published package: release publishes --workspace billion-context-opencode only; @bili/core is private (never published).
• Default branch: master (renamed from main).
• No Chinese README: dropped the README.zh-CN.md changelog check.
• Version source: packages/billion-context-opencode/package.json (not root).

Verification

• bash scripts/ci/check-pr.sh 2026-08-13_project-standards origin/master → PASS ✅
• npm run typecheck --workspaces → clean ✅ (no source touched)
• bash -n scripts/ci/check-pr.sh → syntax valid ✅
• YAML lint (python3 -m yaml) → all 3 workflows parse ✅
• git status → only README.md modified + new AGENTS.md / .github/ / devlog/ / scripts/ — no packages/ changes

Notes

• No version bump (standards-only PR). First real release will follow the §5.4.2 release-branch process.
• NPM_TOKEN secret still needs to be added to repo settings for release.yml to publish (documented in AGENTS.md §5.4.3).
• Branch protection already configured (1 review, enforce_admins: false); should additionally require the build and pr-validation checks.