feat(ambient): add instructions_file for custom system prompt

[chaodu-agent]

Summary

Add support for loading ambient mode system instructions from an external file (~/.openab/config/ambient.md) instead of using the hardcoded default.

Changes

• config.rs: Added instructions_file field to AmbientConfig (default: ~/.openab/config/ambient.md)
• ambient.rs: Added load_instructions() that reads from the configured path (first 2000 chars), falls back to built-in default if file is missing
• docs/ambient.md: Documented the new feature with usage examples
• config.toml.example: Added ambient section with the new field

Behavior

• On startup, reads ~/.openab/config/ambient.md (or custom path from config)
• If file exists: uses first 2000 characters as the ambient system prompt
• If file missing: uses the built-in default (no behavior change for existing users)
• Zero config required — works out of the box with convention-based path

Testing

• Existing unit tests pass (no behavioral change when file is absent)
• Manual: create ~/.openab/config/ambient.md → bot uses custom instructions

[chaodu-agent]

LGTM ✅ — Clean, minimal feature addition with proper fallback behavior and good test coverage.

What This PR Does

Adds support for loading ambient mode system instructions from an external file (~/.openab/config/ambient.md) instead of the hardcoded default, giving operators control over the ambient system prompt without code changes.

How It Works

• New instructions_file field in AmbientConfig (default: ~/.openab/config/ambient.md)
• load_instructions() reads the file at startup, truncates to 2000 chars, falls back to built-in default if missing or unreadable
• The loaded instruction string is stored in AmbientDispatcher and cloned into each channel consumer loop
• Tilde expansion handles ~/ paths; warns if path is outside $HOME

Findings

#	Severity	Finding	Location
1	🟢	Clean fallback chain: file → warn on error → default. All IO errors handled gracefully	ambient.rs:80-97
2	🟢	Good test coverage: exists, missing, and truncation cases	ambient.rs:765-798
3	🟢	Docs clearly state restart-required semantics and provide a working example	docs/ambient.md
4	🟢	Config example is well-commented and follows existing patterns	config.toml.example

Baseline Check

• PR opened: 2026-06-27
• Main already has: hardcoded AMBIENT_SYSTEM_INSTRUCTION constant with no external file loading
• Net-new value: operator-configurable ambient system prompt via external file, with safe defaults and documentation

What's Good (🟢)

• Zero-config default — existing deployments unaffected
• Defensive coding: warns on paths outside $HOME, handles all io::Error variants
• Truncation prevents unbounded memory from large files
• Tests cover the three key paths (happy, missing, truncation)
• Documentation is clear with a practical example