Add startup financial modeling template document

[jardimalegria]

Motivation

• Provide a single, practical 3–5 year financial modeling template to help early-stage startups produce board-ready plans, investor-facing forecasts, and scenario-driven cash/runway analyses.
• Make cohort-based revenue, cost structure, headcount planning, and fundraising mechanics easy to operationalize in a spreadsheet so teams can iterate assumptions quickly.

Description

• Added a new documentation file STARTUP_FINANCIAL_MODEL_TEMPLATE.md that documents model architecture, cohort revenue formulas, cost buckets, headcount planning, cash flow/runway mechanics, KPIs, P10/P50/P90 scenario guidance, and fundraising/dilution integration.
• Included a recommended worksheet/tab layout and a step-by-step quick build sequence to make implementation straightforward in spreadsheets.
• Committed the new file to the current branch and created a PR record via the repository tooling.

Testing

• This is a documentation-only change with no runtime code modifications.
• Verified the commit succeeded via git commit and the working tree is clean using git status --short.
• Created the PR artifact using the make_pr automation.

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Codex Task