docs(arch): add decentralized bot mesh design

[haowei2000]

Capture the converged design for replacing the central coordinator with a decentralized @mention-routed bot mesh:

• no scheduler: channel forwards @, falls back to default_bot, else silent
• per-channel gap-free channel_seq event clock (allocated at finalize)
• two-class resource consistency (Class 1 append-only / Class 2 fs workspace), relaxed to monotonic completeness; read-current + cursor + index recovery
• independent channel_operations log unioned via channel_seq, no fan-out
• Environment template plugin layered above the resource API
• Bot@Bot task chains: dispatch gate (authoritative) + reuse per-msg cancel frame
• opt-in channel-level runaway budget reusing the chain status gate

Design intent ahead of code; includes the gap vs the current gateway and a step-by-step implementation sequence. Indexed from ARCHITECTURE_OVERVIEW.

Change description

Language: English | 中文

Change type

• New features (feat)
• Bug fix (fix)
• Refactor
• Documentation (docs)
• test
• Others: ___

Test

• Unit tests added/updated
• Local pytest passed in full
• Front-end npm run build no error reported
• When it comes to npm packages, the npm run lint / npm test / npm run build of the corresponding package has passed

Database migration

• Does not involve database changes
• Alembic migration file added and verified locally (alembic heads has one head, then alembic upgrade head)

Security and Release Impact

• Does not contain .env, logs, databases, uploaded files, private keys, tokens or production configurations
• Updated documentation when it comes to configuration, deployment, permissions, file uploads, or Agent Bridge

Related Issues

Closes #