feat(v0.7.55): add external content navigation

[qiansc]

v0.7.55

External content navigation

• Declare external knowledge, skills and plugins in imports.yaml without copying, installing or fetching their content.
• Resolve context:import/<id> links to declared URLs and optionally generate an external-content directory in knowledge packages and websites.
• Keep builds usable when optional imports are missing or malformed; retain reader labels and report actionable warnings, including incremental builds.
• Preserve Markdown images, nested links, reference definitions and URL escaping during link projection.
• Expose import guidance through the Context entrypoint, query skills and packaged resources while keeping navigation separate from source evidence.

Validation

• Workspace build, typecheck and lint completed locally (no lint errors).
• Targeted imports, initialization, link projection and real Node CLI scenarios passed during implementation.
• Isolated same-repository and external-navigation integration scenarios passed.
• The release workflow runs verification before publication and full verification afterward.

[qiansc]

Local release validation completed successfully: full workspace build, verify:full (static checks, default tests, full-only integration tests, Node dist smoke checks and real CLI artifact tests), and dry-run package audits for all 13 release packages. Publication is running through the release workflow at https://github.com/context4ai/context/actions/runs/37193877854.