docs: rewrite README for first success

[JNHFlow21]

Summary

• rewrite the README around one clear promise: define each MCP once and store each credential once
• move the verified source-install path and observable first success ahead of the feature catalog
• document local credential boundaries, supported integrations, and alpha limitations without presenting roadmap work as shipped
• update the docs regression test to preserve the new quick-start-before-capabilities structure

Verification

• PYTHONPATH=src python3 -m unittest discover -s tests -v (72 passed)
• PYTHONPATH=src python3 -m unittest discover -s tests/integration -v (1 passed)
• isolated pipx install from a fresh GitHub clone; agent-switch --version returned 0.2.0
• swift build --package-path macos-app/AgentSwitch
• Xcode Release build with signing disabled
• python3 scripts/privacy_scan.py
• README strict audit: 100/100, no findings
• all relative README links resolved

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Document the source-launch override

The development section no longer explains how to make a locally built app use the checkout. After following the shown venv setup, launching the app against source will not pick that environment: AgentSwitchService only enables source execution when AGENT_SWITCH_DEV_ROOT is set; otherwise it looks for an installed CLI and then invokes a system Python without the checkout's PYTHONPATH. CONTRIBUTING.md does not supply this missing runtime instruction, so developers cannot reliably exercise their source changes in the native app.

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