docs: reapply agent docs, testing infra, and sandcastle fixes

[krake747]

• AGENTS.md: rewrite for React 19 stack, layer table, verification chain
• .sandcastle/CODING_STANDARDS.md: remove SolidJS refs, add React/TanStack Query/Base UI patterns
• .sandcastle/linear: add --project flag and resolve_project_id helper
• .sandcastle/plan-prompt.md: scope queries to Next Bite project
• .sandcastle/main.ts: remove redundant gh auth setup-git hook
• docs/agents/testing.md: new — Vitest/browser/E2E testing conventions
• docs/agents/toolchain.md: new — build/lint/test/sandcastle commands
• vitest.config.ts, src/test-setup.ts: new — Vitest config with jsdom, aliases
• src/core/image-utils.test.ts: sample unit tests (6 tests, passing)
• src/ui/badge.test.tsx: sample browser tests (4 tests, passing)
• .oxlintrc.json: add test file overrides for vitest rules
• package.json: add test, test:watch, test:all scripts

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📥 Commits

Reviewing files that changed from the base of the PR and between 5d25728 and a346461.

⛔ Files ignored due to path filters (1)

• pnpm-lock.yaml is excluded by !**/pnpm-lock.yaml

📒 Files selected for processing (13)

• .oxlintrc.json
• .sandcastle/CODING_STANDARDS.md
• .sandcastle/linear
• .sandcastle/main.ts
• .sandcastle/plan-prompt.md
• AGENTS.md
• docs/agents/testing.md
• docs/agents/toolchain.md
• package.json
• src/core/image-utils.test.ts
• src/test-setup.ts
• src/ui/badge.test.tsx
• vitest.config.ts

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