docs: design shared AI connections with stable executable routes

[atomantic]

Summary

Design shared backend connections and harness bindings while preserving executable provider IDs, saved model selections, mode-specific settings and older-client behavior. The decision includes concrete storage/API examples, explicit link/unlink flows, migration and downgrade reconciliation, and a validation matrix.

Filed implementation slices #6366, #6367, #6368 and #6369 with explicit dependencies. This PR completes planning only; no provider runtime or storage behavior changes.

Test plan

• Parsed both JSON examples and verified all 24 referenced source paths.
• Checked whitespace and scanned the documentation for private-data patterns.
• CI impact planner classified the diff as documentation-only and selected repository guards.
• Claude reviewed the diff with tools disabled, plan permissions and low effort; addressed binding identity, revision, deletion-recovery and documentation findings.

Closes #6359