docs: add repository structure and contributor guidelines

[ZouR-Ma]

Problem

The repository's agent instructions covered commit attribution but did not provide a shared map of module ownership, dependency boundaries, coding practices, or collaboration and validation expectations.

Changes

• Add 15 English AGENTS.md files covering the repository, packages, CLI host and UI, generic harness, documentation, scripts, release packaging, and plugins.
• Map the actual project directories and the owners of Agent Loop, sessions, context, providers, tools, configuration, orchestration, themes, and host protocols. Keep coding-agent subdirectory guidance in its package-level file.
• Add development and testing guides for types, abstraction, errors, asynchronous cleanup, contributor ownership, interface handoffs, fixtures, consumer-level tests, and validation evidence.
• Require English Conventional Commit titles and English collaboration text; add a PR template and review checklist for compatibility, model-visible behavior, affected consumers, and verification.
• Preserve the existing attribution policy and CLAUDE.md reference, and keep private release infrastructure outside the public repository.

Behavior and compatibility

Repository coding agents receive directory-specific implementation and collaboration rules, and contributors receive consistent submission and review guidance. Product runtime behavior, product prompts, dependencies, and public package interfaces are unchanged.

Validation

• pnpm run check passed on macOS with Node 24.3.0 and pnpm 9.15.9, including types, public-source boundaries, repository guards, and browser bundle smoke.
• ./test.sh passed with its isolated user-resource environment, TMPDIR=/private/tmp, and a local wrapper for the existing pnpm 9.15.9 cache. Coding-agent: 4,435 passed / 59 skipped; CLI: 677 passed; the other workspace suites and root script tests also passed. No real model requests were enabled.
• git diff --check 3ba89e0cbfe6be78cccdafb5aeafd2592177cbb8...HEAD passed.
• Checked 87 relative links, all 35 directories in the project structure, and 14 documented test entry points against the checkout. Confirmed the original attribution policy and CLAUDE.md were preserved.
• GitHub CI passed for a90bfa5: Linux / Node 22 build-check-test and Check commit attribution both completed successfully.

Limitations

Local execution was on macOS, with Linux / Node 22 covered by GitHub CI. Windows-specific execution was not validated; platform-dependent tests retain their existing skips.