[codex] Fix node-scoped Linear disconnect

[rhymiz]

Summary

• Add node-scoped Linear connection links so disconnecting one Foreman server node does not disconnect other nodes that share the same Linear workspace credential.
• Send the selected node id on desktop proxy requests and enforce active node-link checks before node-scoped Linear operations can use or refresh credentials.
• Revoke shared Linear workspace credentials only after the last active node link is removed, and update docs for the node-scoped disconnect behavior.

Root Cause

Linear connection rows were keyed around the Foreman user and Linear workspace, so disconnecting a workspace revoked the shared credential row. Multiple Foreman server nodes owned by the same user could therefore lose access when only one selected node was disconnected.

Validation

• cargo test -p foreman-proxy
• cargo test --workspace
• cargo fmt --check
• cd foreman-desktop && bun run typecheck
• git diff --check