refactor(cli): extract parser diagnostics

[kunaldhongade]

Refs #262

Scope

Fourth small CLI modularization slice under the package-by-package refactor roadmap. This moves unknown command/option diagnostics and suggestion matching out of packages/cli/src/index.ts into packages/cli/src/parsers/diagnostics.ts.

What changed

• Added packages/cli/src/parsers/diagnostics.ts for unknown-command and unknown-option error construction.
• Moved suggestion token normalization and Levenshtein matching out of index.ts.
• Kept command routing, argument parsing, handlers, rendering, runtime errors, and output behavior in index.ts.
• Preserved current user-facing error text and exit code behavior.
• Reduced packages/cli/src/index.ts from about 7,498 lines to about 7,397 lines.

Safety

• Behavior-preserving refactor only.
• No command names, flags, stdout/stderr format, or exit-code changes intended.
• No telemetry, cloud dependency, API key requirement, hidden LLM/model call, or CodeDecayCloud dependency added.
• No generated local artifacts or user data committed.

Validation

• pnpm typecheck
• pnpm run lint
• pnpm exec vitest run packages/cli/test/cli.test.ts packages/cli/test/built-cli.test.ts packages/core/test/core.test.ts
• pnpm test
• pnpm build
• pnpm --filter @submuxhq/codedecay pack --dry-run
• node packages/cli/dist/index.js analze exits 2 and suggests analyze
• node packages/cli/dist/index.js analyze --formt markdown exits 2 and suggests --format
• node packages/cli/dist/index.js --help
• node packages/cli/dist/index.js version