docs: explain Community CLI canaries and detection use cases

[andy-smith-tracebit]

Description

The Community CLI README previously listed supported canaries and installation commands but gave little context about what triggers an alert or when to use the tool.

• Add explanations of credential canaries, supported deployment locations, and use cases involving stolen credentials and AI agents.
• Include approved anonymised detection examples and link to Synthesia's published experience and Tracebit's Context Bombs research. Cloud infrastructure examples are explicitly distinguished from workstation CLI capabilities.
• Keep account creation, installation, and deployment instructions near the top. Preserve the release badge, latest-release link, API documentation, OpenAPI specification, and MIT license link.
• Describe the actual deployment workflow: local AWS and SSH files, browser-assisted cookie placement, manual password-manager placement, canary emails, and background refresh for supported types.
• Clarify that credential alerts require attempted use and that an absence of alerts does not prove an agent stayed within scope.

Only the README changes; CLI behavior is unchanged.

[github-actions]

Package	Line Rate	Branch Rate	Health
tracebit	40%	33%	➖
Summary	40% (2895 / 7183)	33% (354 / 1070)	➖