Implement RunwayBot.fetch_signals with demo fixtures

[askmy-stack]

Background

Identified during OSS audit v2 of startupintel.

Problem Statement

NotImplementedError.

Proposed Solution

DEMO_MODE fixtures.

Acceptance Criteria

• Implementation complete
• Tests pass
• Documented in README or docs/

Files / Modules Affected

startupintel/bots/runway_bot.py

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Suggested labels: priority:high, difficulty:intermediate, category:feature
Milestone: v2.0-features
OSS Audit: Master Backlog v2