feat: show config settings and AI prompt in metadata output

[maximgorbatyuk]

Summary

• The metadata command now displays all configuration settings after the existing database/hooks info: AI provider, model, API key (masked), CLI preference, prompt instructions, and auto-update flag
• When no custom prompt is configured, the full default prompt is shown (labeled default); when a custom one is set via ai.prompt_instructions, it is displayed in full (labeled custom)
• API keys are masked for security — only first 4 and last 4 characters are visible (e.g. sk-a…cdef)
• Extracted the default prompt instructions into a DEFAULT_PROMPT_INSTRUCTIONS constant in src/ai/mod.rs, reused in both build_prompt and metadata display

Example output

Database size:   1.25 MB
Total commits:   42
Oldest commit:   2026-01-15 09:30:00

Global hooks:    /Users/me/.config/diddo/hooks (ok)
Local hooks:     not set

AI provider:     anthropic
AI model:        claude-sonnet-4-6
AI API key:      sk-a…cdef
CLI preference:  api
Prompt (default):
Using only the commit data provided, produce a concise repository-based summary of changes.

Format:
...
Guidelines:
- Group commits by repository.
- Convert commit messages into short, clear descriptions of what changed.
...

Auto-update:     true

Test plan

• All 201 existing tests pass
• Clippy clean
• New tests: metadata_shows_config_settings, metadata_shows_defaults_when_config_is_empty, mask_api_key_hides_middle_of_long_keys

[drugoi]

@cursor let's check this PR

[cursor]

Here is what I did for “let’s check this PR” on extended-metadata / #22.

Review

• Metadata feature matches the description: format_metadata appends resolved AI settings, masked API keys (mask_api_key), default vs custom prompt via ai::DEFAULT_PROMPT_INSTRUCTIONS, and update.auto_check.
• Tests (metadata_shows_config_settings, metadata_shows_defaults_when_config_is_empty, mask_api_key_hides_middle_of_long_keys) line up with that behavior.

Verification

• cargo test — 201 passed
• cargo fmt -- --check — clean
• cargo clippy --all-targets --all-features — initially 2 warnings in src/interactive.rs tests (clippy::field_reassign_with_default), unrelated to metadata but worth fixing for strict CI.

Change pushed

• Updated two range-form tests to use RangeFormState { from: ..., ..Default::default() } instead of default() then assigning from.
• Commit: style: fix clippy field_reassign_with_default in interactive tests — pushed to origin/extended-metadata.

I could not update the GitHub PR description from here (platform reported the body is not agent-managed). You can paste a short note manually if you want the test plan to mention clippy --all-targets is now warning-free.