A `--field github_pr=...` / MCP `field` setter stored the PR as a string on
every transport, so no link rendered, and `=null` stored the string "null".
BUG-2627 had exempted github_pr from the reserved-key refusal as remote MCP's
only writer; that writer never worked.
ItemUpdate gains a typed `github_pr` PR object (number > 0 and an absolute
http(s) url required) plus `clear_github_pr`, lowered by the handler into the
field patch after the caller's own fields_patch is refused every reserved key.
`pad github link` / `unlink` / `project reconcile` use it and verify from the
response that the change landed (an older server ignores the member and
answers 200). Not exposed on the MCP catalog; a remote PR-link action is
deferred to the catalog-trim decision. The point is validity, not sender
identity. Create (still accepts hand-written reserved keys) is BUG-3163.
Claude-Session: https://claude.ai/code/session_01RYiqbqXmxQMoKHsGm8B77N
xarmian