fix(config): add #[serde(default)] to partial config sections

[CtriXin]

Summary

Partial TOML config sections like [limits] now parse correctly when only some fields are specified.

Root cause: Without #[serde(default)], missing fields in a TOML section cause toml::from_str to return a missing field error. This bubbles up to Config::load(), which silently falls back to full defaults — discarding every user-specified override.

Fix: Added #[serde(default)] to five sub-config structs: TrackingConfig, DisplayConfig, FilterConfig, TelemetryConfig, LimitsConfig. Each already implements Default with correct values, so serde now falls back to those defaults for any missing fields.

Fixes: #1482

Test Plan

• cargo test config — 14 tests pass
• Manual: [limits] with only grep_max_results = 500 applies correctly

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[pszymkowiak]

[w] wshm · Automated triage by AI

📊 Automated PR Analysis

🐛 Type	bug-fix
🟢 Risk	low

Summary

Adds #[serde(default)] to five partial config structs (TrackingConfig, DisplayConfig, FilterConfig, TelemetryConfig, LimitsConfig) so that TOML sections with only some fields specified parse correctly instead of causing a missing field error that silently falls back to full defaults, discarding user overrides.

Review Checklist

• Tests present
• Breaking change
• Docs updated

Linked issues: #1482

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Analyzed automatically by wshm · This is an automated analysis, not a human review.