fix(ambient): default allow_bot_messages to true

[chaodu-agent]

Bot messages should be included in the ambient buffer by default so the agent has full channel context when deciding whether to reply. Users can opt out with allow_bot_messages = false if needed.

[chaodu-agent]

LGTM ✅ — Clean default flip with consistent doc updates.

What This PR Does

Changes the default value of allow_bot_messages in AmbientDiscordConfig from false to true, so bot messages are included in the ambient buffer by default — giving agents full channel context without extra configuration.

How It Works

• Removes the derive Default and implements Default manually to set allow_bot_messages: true
• Uses the existing default_true() serde helper for deserialization consistency
• Updates both docs/ambient.md and docs/config-reference.md to reflect the new default

Findings

#	Severity	Finding	Location
1	🟢	Reuses existing default_true() helper — no unnecessary additions	config.rs
2	🟢	Manual Default impl is minimal and correct	config.rs:1337-1343
3	🟢	Documentation updated consistently across all references	ambient.md, config-reference.md

Baseline Check

• PR opened: 2026-06-27
• Main already has: AmbientDiscordConfig with allow_bot_messages defaulting to false via derive Default
• Net-new value: Flips the default to true so agents passively observe bot messages without opt-in config

What's Good (🟢)

• Minimal diff — only touches what needs to change
• Leverages existing default_true() helper at line 824, no code duplication
• Both serde deserialization default and struct Default impl agree on the same value (true), preventing config inconsistencies