fix(oxidize): add defensive check for missing co_changes table

[nicabarnimble]

Problem

oxidize crashes with no such table: co_changes when git history data isn't available. This happens when:

• scrape git was never run
• Shallow clone detected and skipped
• New repo with insufficient commit history

Current Behavior

// temporal.rs:29 - crashes if table missing
conn.prepare("SELECT file_a, file_b, count FROM co_changes ...")?;

Error: no such table: co_changes

Proposed Fix

Add table existence check before querying:

// In oxidize, before querying co_changes:
let table_exists: bool = conn.query_row(
    "SELECT EXISTS(SELECT 1 FROM sqlite_master WHERE type='table' AND name='co_changes')",
    [],
    |row| row.get(0)
)?;

if !table_exists {
    println!("⚠️  Skipping temporal projection (no git history data)");
    return Ok(vec![]); // Skip gracefully instead of crash
}

Additional Edge Cases to Handle

Case	Current	Proposed
Table missing	Crash	Skip with warning
Table empty	Bail	Skip with warning
Few co-changes (count < 2)	Bail	Skip with warning
All single-file commits	Empty pairs, bail	Skip with warning

Context

Discovered in PR #67 when CI failed due to shallow clone. Fixed CI with fetch-depth: 0, but oxidize should be resilient regardless of git history availability.

Labels

• enhancement
• good first issue