feat(cli): wire 'why' command

[quarnel-dev]

Closes #1.

Wires the existing handleWhy handler (src/tools/why.ts) as a CLI command
in src/index.ts. Mirrors the signals pattern: load config -> store ->
handler -> print.

• contextix why "<event>" — causal chain
• contextix why "<event>" --depth N — depth limit (default 3)
• contextix why "<event>" --json — JSON output
• contextix why --help — usage

Tradeoffs: none. Reuses traceCausalChain as-is, no changes to query layer.

Tested locally:

• node dist/index.js why --help — usage printed
• node dist/index.js why "record-demo" — chain returned
• node dist/index.js why "record-demo" --depth 2 — depth honored
• node dist/index.js why "record-demo" --json — MCP-shaped JSON

Note: local regex graph after ingest markdown . has no multi-hop causal
chains, so chainLength is 1 for all events tested. CLI wiring works.

npm run typecheck and npm run build pass.