Add LLM-friendly non-interactive snapshot management

[max-sixty]

From Claude

Summary

Enhance cargo-insta for use in non-TTY environments (LLMs, CI pipelines, scripts) by adding non-interactive modes for snapshot review and management.

Changes

Non-Interactive Review Mode

• cargo insta review --snapshot <path> now works without a TTY
• Shows the snapshot diff in a read-only mode
• Provides instructions for accepting/rejecting

Non-Interactive Reject Mode

• cargo insta reject --snapshot <path> now works without a TTY
• Shows the diff before rejecting (so users can verify what they're rejecting)
• Actually performs the rejection

Enhanced pending-snapshots Output

• Now shows helpful usage instructions instead of just paths
• Provides example commands with actual snapshot paths
• Uses workspace-relative paths for better readability

Helper Function

• Extracted format_snapshot_key() to eliminate code duplication
• Consistently uses workspace-relative paths throughout

Improved Error Messages

• Updated TTY error message to guide users to non-interactive alternatives
• Mentions all available non-interactive commands

Examples

Before (without TTY):

$ cargo insta review
error: Interactive review requires a terminal. Use `cargo insta accept` or `cargo insta reject`...

After (without TTY):

$ cargo insta pending-snapshots
Pending snapshots:
  src/lib.rs:42
  src/snapshots/test__example.snap

To review a snapshot: cargo insta review --snapshot 'src/lib.rs:42'
To accept a snapshot: cargo insta accept --snapshot 'src/lib.rs:42'
To reject a snapshot: cargo insta reject --snapshot 'src/lib.rs:42'

$ cargo insta review --snapshot 'src/lib.rs:42'
Snapshot: src/lib.rs:42 (inline):
  Package: example@0.1.0

[... shows full diff ...]

To accept: cargo insta accept --snapshot 'src/lib.rs:42'
To reject: cargo insta reject --snapshot 'src/lib.rs:42'

$ cargo insta accept --snapshot 'src/lib.rs:42'
insta review finished
accepted:
  src/lib.rs:42 (inline)

Use Cases

This is particularly useful for:

• LLM/AI coding assistants - Can now review and manage snapshots programmatically
• CI/CD pipelines - Scripts can review specific snapshots without interactive prompts
• Automated workflows - Tools can inspect snapshot diffs before deciding to accept/reject

Testing

• ✅ All existing tests pass (62 tests)
• ✅ All lints pass (pre-commit run --all-files)
• ✅ Manually tested non-interactive review/reject workflows
• ✅ Verified workspace-relative paths work correctly

Backward Compatibility

• All changes are additive - existing behavior unchanged
• Interactive mode still works as before
• JSON output from pending-snapshots --as-json maintains absolute paths for machine consumption
• Human-readable output uses relative paths for better UX