docs(features): organize capabilities around five systems

[futrime]

Summary

• reorganize the Feature information architecture around five reader-facing systems and nineteen capability groups
• add a complete capability map plus interactive deep dives for prophecy compilation, detached runs, and shared product surfaces
• clarify boundaries across ordinary flows and atlases, resumption, backend capabilities, tracing, telemetry, credentials, and remote execution

Verification

• corepack pnpm build
• corepack pnpm check:anchors
• uv run pre-commit run --all-files
• targeted feature tests: 323 passed, 1 skipped

Full uv run pytest was not completed locally: on this macOS arm64 host, collection stops in tests/coganchor/test_redirect.py and tests/coganchor/test_smoke.py when the x86_64-only coganchor interception layer is imported. The full suite runs in CI on Ubuntu x64 for Python 3.12–3.14.

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