docs: information-architecture pass — organize docs around the control-plane model

[williamzujkowski]

Part of #3858. Depends on #3859.

Context

docs/ reorganization so the control-plane metaphor is navigable (entry point → admission control/gates → event log/audit → data plane/agents → self-* loops). Must NOT conflict with the #3532/#3688/#3763 single-sourcing work — coordinate: generated references stay where the generators emit; this pass owns the narrative docs around them. The #3163 user-journey residuals are input.

Acceptance criteria

• docs/README.md index restructured; narrative docs mapped to control-plane sections; no broken links (links gate)
• Redirect/alias notes for moved files; docs gates green
• Explicit non-conflict note with the docs-site epic (Epic: single-source the docs site + tool reference from JSDoc/code (Astro + TypeDoc) — eliminate doc drift #3532)

Evidence required

Docs gates (links, index, markdownlint).

Out of scope

Generated API reference (owned by #3532/#3688/#3763).