feat: diff JSON output for project diffs and --fail-on-drift CI gate

[FIERsity]

What

• diff --project-a/--project-b now honors --format json — machine-readable drift reports with sensitive values redacted unless --show-values
• New --fail-on-drift flag on both diff modes: exits 1 when any variable carries different values, turning drift detection into a CI gate

Why

Project diff only had human output; CI consumers had no machine-readable format and no exit code to fail on. This closes the gap with the same conventions as audit (--format json, fail-to-exit semantics).

Verification

• 5 new integration tests (diff JSON redaction, fail-on-drift failure/success for both files and project modes)
• cargo test 116 green, clippy clean, fmt clean

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📥 Commits

Reviewing files that changed from the base of the PR and between 20478a2 and 037347d.

📒 Files selected for processing (6)

• CHANGELOG.md
• README.md
• src/cli.rs
• src/diff.rs
• src/main.rs
• tests/cli.rs

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