[MPDX-9796] - Show the real attendee fields and make the coach editable

[wjames111]

Description

Staff Details goal settings showed Coach, Coordinator, Training and Ministry as hardcoded text
("Amy Wilson", "Fall NSO 2026", "Campus: University", three invented coordinator names). This
replaces all four with the household's real values, and makes the coach assignable, changeable and
removable from the page.

• Load newStaffCohortAttendee with the goal calculation. It is null for scenario goals and for
  anyone outside the MPD Goals team, so the staff member's own wizard, which shares this query
  document, is unaffected.
• Training, Ministry and Coordinators now render real values. All three stay read-only, since they
  are owned by the attendee and OneApp.
• Coach gains Assign / Change / Remove. The picker is the admin table's existing AssignCoachModal,
  so both places offer the same list, and it loads lazily since most visits never open it. Remove
  asks for confirmation first and says the coach loses access to the account.
• GoalSettingsPlaceholder is renamed to GoalSettingsReadOnlyField. Same UI, but it is no longer a
  stand-in for missing data, so its docstring no longer says it is one.

Depends on CruGlobal/mpdx_api#3601. That PR adds newStaffCohortAttendee, cohortName,
newStaffCohortId, the attendeeId argument on newStaffCohortAssignableCoaches and the
unassignCoachFromNewStaffCohortAttendee mutation. Codegen resolves the schema from the API, so
CI here will fail until #3601 is merged and deployed to staging. Do not merge before then.

• https://jira.cru.org/browse/MPDX-9796

Testing

• Open the MPD Goal Admin table and click View/Edit on a household to reach Staff Details
• Check the Coach, Coordinators, Training and Ministry values match that household's row in the table
• On a household with no coach, click Assign Coach, pick one, and save; the coach appears without a reload
• Click Change, pick a different coach, and save; the new coach appears
• Click Remove, confirm, and check the coach clears
• Open a staff member's own goal calculator and check it still loads (they get no household)

Checklist:

• I have given my PR a title with the format "MPDX-(JIRA#) (summary sentence max 80 chars)"
• I have applied the appropriate labels (Add the label "Preview" to automatically create a preview environment)
• I have run the Claude Code /quality:agent-review command locally and fixed any relevant suggestions
• I have requested a review from another person on the project
• I have tested my changes in preview or in staging
• I have cleaned up my commit history