Agent M4: workspace agent rail primacy and legacy AI surface demotion

[cevheri]

Goal

Make the agent rail the single primary AI surface of Studio and demote the legacy AI panels, without breaking platform/embed consumers.

Scope

• Feature flag flips to default-on for standalone builds; embedded builds keep the agent invisible via the capability prop
• BottomPanel NL2SQL / Autopilot tabs become secondary shortcuts that open the Agent with a prefilled workflowType + prompt (NL2SQL -> investigation draft, Autopilot -> database-assessment); they are no longer marketed or laid out as equal AI products
• Editor inline chat routes to the agent rail
• Capability-fallback surface: models failing the probe show an explicit "Agent unavailable on this model - falling back to chat/NL2SQL" state that can open the legacy panel
• src/lib/llm/ and src/app/api/ai/* stay functional (platform/embed consumers + fallback); new agent routes do not layer a second abstraction over them
• Sunset gate documented: once the agent covers the NL2SQL + Autopilot happy paths in eval, remove/merge those tabs into a single "Ask Agent" shortcut; API cleanup tracked separately
• Docs: agent user guide, security posture note (read-only R0/R1, policy pipeline), README feature section
• Docs: single-page data-flow document - what leaves the machine, when, to which provider, and where redaction applies (continuation of the security posture page; a DRAFT of this page must already ship with the M2 preview)
• Ollama as a documented first-class default path: local-model setup guide and default-model recommendation in the agent docs ("the agent runs next to your data - even the model can")
• Public workspace props extended additively/optionally only; build:lib + packed tarball + attw prove the embed contract

Acceptance criteria

• Standalone: rail is the primary entry; legacy tabs open the agent prefilled.
• Embedded: zero agent UI, zero new required props, no layout/style regressions in platform mode.
• Fallback state reachable and honest on an incapable model.
• Eval evidence recorded for the sunset gate decision.

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Part of #325