docs: add sync protocol guide (DOC-06)

[bertheto]

Summary

Add docs/guides/sync-protocol.md — a comprehensive guide to the AgentChatBus sync protocol.

Content

• How It Works — sequence diagram (Mermaid) of the full sync cycle: context issue, msg_post, success/error, recovery
• Sync Context — table of current_seq, reply_token, reply_window fields
• Reply Tokens — lifecycle flowchart (Mermaid), token sources table, validation rules
• Sequence Validation — SEQ_TOLERANCE formula and behaviour
• Fast-Return Logic — 3-level priority table (messages, bus_connect token, sync-only)
• Error Types — 5 sync exceptions table with conditions
• Error Response Shape — full JSON example with CRITICAL_REMINDER and new_messages_1st_read
• Recovery Flow — flowchart (Mermaid) + 4-step procedure
• Configuration — AGENTCHATBUS_SEQ_TOLERANCE and AGENTCHATBUS_SEQ_MISMATCH_MAX_MESSAGES
• Best Practices — agent-facing guidance
• See Also — links to bus-connect guide and tools reference

mkdocs.yml changes

• Enable Mermaid rendering: pymdownx.superfences custom_fences + extra_javascript: unpkg mermaid@10
• Add nav entry: Guides > Sync Protocol

Test plan

• mkdocs serve builds without errors
• Sequence diagram renders correctly (How It Works)
• Lifecycle flowchart renders correctly (Reply Tokens)
• Recovery flowchart renders correctly (Recovery Flow)
• Nav entry visible under Guides
• All internal links resolve (note: bus-connect.md link resolves after PR docs: add bus_connect guide (DOC-01) #46 merges)