Server names: display name and host name on requests, briefs and webhook tokens (#5632 Phase C C3) #5999

Claude posting for Erik Darling.
Part of #5632 (Phase C, wave 1).

What it does

Requests, briefs and the webhook token carry both display_name and host_name. They are read from the registry when served, so a rename shows on the next read. server_name keeps its meaning (the display name) for everyone who reads it today.

Decisions this slice carries (agreed by Erik on 2026-10-16)

• D18 (A). "display_name and host_name on the request, the brief and the webhook token, read from the registry when served." B (stored at open) was rejected because it "goes stale on a rename." "server_name keeps its meaning."

Files to touch

Pointers re-checked on dev at c12233ce3.

• PerformanceMonitor.Common/Investigations/InvestigationToolCore.cs:466-472 (RequestFields, server_name = r.ServerName at 471).
• The registry is the servers table (Darling/PerformanceMonitor.Darling.Storage/PgMigrations.cs:4051-4060: server_name and display_name). Existing display-name read: MonitoredServerDisplayNameAsync at Darling/PerformanceMonitor.Darling.Service/Mcp/DarlingMcpAlertTools.cs:1206.
• Darling/PerformanceMonitor.Darling.Storage/InvestigationRequestStore.cs:98 (the server_name the request is opened with), :169, :177, :257 (the reads that return it).
• PerformanceMonitor.Common/Investigations/InvestigationBriefComposer.cs (the brief) and PerformanceMonitor.Common/Investigations/InvestigationWebhookToken.cs (Render, called from PerformanceMonitor.Notifications/InvestigationDispatchCore.cs:236 and :245).
• Pointer fixed: the plan cited DarlingWorker.cs:14571 for the request's server name. That line is in the collector snapshot loop and has nothing to do with it; the open path runs through Darling/PerformanceMonitor.Darling.Service/InvestigationDispatcher.cs:164-168 into the store at InvestigationRequestStore.cs:98.
• No migration (the plan says read when served). If a join is needed for the MCP and token reads, put it in the existing request reads.

Depends on

None.

Behaviour to pin

• A renamed display name shows on the next read of an existing request.
• The host name matches the registry.
• server_name still returns the display name.

Tests

Behaviour pins only, one test per behaviour listed above. No census, roster or source-text test (the #5833 ruling in .github/REVIEW-TRAPS.md).

Notes

• Lite's equivalent comes in Lite: recurrence, server names and report fields (#5632 Phase C L2) #6011.
• Nothing under deprecated/.
• At most four parts. If the work needs more, stop and say so in the PR body rather than growing the slice.
• Report the changelog entry in the PR body (the coordinator splices it after the wave; do not edit CHANGELOG.md).