Add baseline command for migration cleanup

[rwdaigle]

Summary

Adds the migrate baseline <version> command which marks a specific version as the baseline, allowing old migration files to be deleted while preserving history. This enables migration cleanup and reduces repository bloat over time.

• New migrate baseline <version> command to mark versions as applied
• --baseline and --keep flags on migrate up for inline baselining
• .baseline file tracks baseline version and optional summary
• Documentation updated in README.md and CLAUDE.md
• Version bumped to 0.4.0

Test plan

• All 48 existing tests pass
• New baseline functionality tested via unit tests in src/baseline.rs
• Version consistency enforced through Cargo.toml

🤖 Generated with Claude Code

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[devin-ai-integration]

Devin Review found 1 potential issue.

View issue and 5 additional flags in Devin Review.

[devin-ai-integration]

🟡 Malformed migration IDs incorrectly marked as baselined due to Option comparison

In the status command, when checking if a migration is baselined, the code compares extract_version(&migration.id) <= Some(b.version.clone()). The extract_version function returns None for malformed migration IDs (those that don't have a dash at position 5).

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Issue Details

In Rust, None < Some(_) is always true due to how Option ordering works. This means if a migration ID in the .history file is malformed (e.g., manually edited), the comparison None <= Some(baseline_version) would evaluate to true, causing that migration to be incorrectly displayed with the (baseline) marker.

// src/commands/status.rs:91-93
let is_baselined = baseline
    .as_ref()
    .is_some_and(|b| extract_version(&migration.id) <= Some(b.version.clone()));

The extract_version function at lines 123-129 returns None when the ID doesn't match the expected format:

fn extract_version(id: &str) -> Option<String> {
    if id.len() >= 5 && id.chars().nth(5) == Some('-') {
        Some(id[..5].to_string())
    } else {
        None
    }
}

Impact

This is a display-only bug affecting the migrate status output. It doesn't affect actual migration behavior or data integrity.

Recommendation: Change the comparison to handle the None case explicitly, e.g., extract_version(&migration.id).map_or(false, |v| v <= b.version)

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