Redesign snapshot-diff plugin to compare persisted snapshots

[saurabhsharma2u]

Motivation

• Replace the prior file-based graph JSON comparison flow with a snapshot-aware workflow that compares persisted Crawlith snapshots by ID.
• Align the plugin with the AI Agent plugin guidelines and project architecture by separating CLI, lifecycle hooks, service logic, types, and output formatting.
• Keep and improve incremental crawl behavior so crawls can preload the latest completed snapshot graph for incremental diffing.

Description

• Reworked the plugin into a manifest plus src/ modules: src/cli.ts, src/plugin.ts, src/Service.ts, src/Output.ts, and src/types.ts, and updated index.ts to register the new CLI and hooks.
• Replaced --compare <files...> with --compare <snapshots...> which validates exactly two numeric snapshot IDs via SnapshotDiffService.parseCompareRequest and compares graphs with loadGraphFromSnapshot + compareGraphs from core.
• Implemented presentation helpers renderPrettyDiffOutput and renderJsonDiffOutput to produce colorized terminal summaries or raw JSON depending on --format.
• Improved incremental baseline logic in SnapshotDiffService.resolvePreviousGraph so --incremental injects the previous completed snapshot graph into ctx.metadata.previousGraph and logs context-aware messages.
• Updated tests and docs: expanded tests/snapshotDiff.test.ts to cover incremental loading, compare/terminate flow, and invalid input validation, and revised README.md and package.json description to document usage and output.

Testing

• Built core to ensure exports are resolvable with pnpm --filter @crawlith/core build, which completed successfully.
• Built the plugin with pnpm --filter @crawlith/plugin-snapshot-diff build, which completed successfully (TypeScript validation passed).
• Ran the plugin test suite with pnpm --filter @crawlith/plugin-snapshot-diff test, and all tests passed (4 tests passed).

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Codex Task