feat: add deterministic DCF calculator

[derspotter]

Summary

• add a deterministic dcf_calculator tool for projected FCFs, Gordon Growth terminal value, PVs, fair value per share, and sensitivity tables
• wire the tool into the registry so agents can call it during valuation work
• update the DCF skill to require calculator-backed arithmetic instead of manual terminal value math
• add regression coverage for the Apple owner-earnings case that catches the terminal value mismatch

Testing

• bun run typecheck
• bun test
• bun test src/tools/valuation/dcf-calculator.test.ts