Disclose stability and capability gaps beyond feature packs

[jonathanKingston]

Context

#1367 makes feature-pack stability explicit and derives experimental default-off behavior from the manifest. The product definition-of-done audit applies the same requirement to model strategies, external agents, and unfinished remote capabilities.

Goal

Before a user commits work, every non-stable product surface should disclose its status, capability gaps, data/safety boundary, fallback behavior, and durability implications.

Acceptance criteria

• Inventory user-selectable models/strategies, ACP agents, remote capabilities, and other experimental surfaces.
• Define one shared stability vocabulary and presentation contract where possible.
• Show capability gaps and fallback behavior before selection or enablement.
• Ensure experimental surfaces default safely and preserve user-owned history/data when disabled.
• Add focused component/e2e coverage for the selection and degraded-state paths.
• Link each remaining exception from docs/product-release-evidence.md.

Related

• Declare feature pack stability #1367
• Add capability release-evidence register to product audit #1368
• Model classifier: automatically pick the best model for a given problem #557
• Add A2A protocol support for durable remote-agent tasks and delegation #1015
• ACP terminal/*: client-owned shell with native sandbox + unsandboxed retry #623