Replace inline errorMessage duplicates in the server

[selfcontained]

What

apps/server/src/shared/lib/error-message.ts is the canonical helper for turning an unknown catch value into a string:

exportfunctionerrorMessage(error: unknown): string{returnerrorinstanceofError ? error.message : String(error);}

18 sites across 11 server modules had re-inlined that exact expression instead of importing it. This replaces them with errorMessage(...):

File	Sites
release-checks.ts	4
server/release-runtime.ts	3
jobs/service.ts	2
update-migrations.ts	2
agents/manager.ts (already imported the helper)	1
assisted-update.ts	1
observability/subsystem-tracker.ts	1
personas/authoring.ts	1
release-info.ts	1
routes/release.ts	1
shared/mcp/tool-error.ts (inner branch of a longer chain)	1

Why this is tech debt

This is a regression of the consolidation landed in #608. The tech-debt Brain has a standing pattern note that this exact helper "does not stay fixed on its own" — every new try/catch is an opportunity to retype the ternary instead of adding the import. The duplication is harmless in isolation but it is the reason nobody can change the helper's behavior in one place.

Scope / what was deliberately left alone

Only byte-equivalent occurrences were replaced. A repo-wide grep for instanceof Error found 19 other sites that look similar but are not the same function, and none of them were touched:

• Different fallback string — "Unknown error" (notifications/slack.ts, routes/release.ts:465,486, release-info.ts:243), "Unknown error." (server/http-helpers.ts), "Archive failed" (agents/archive.ts), "Invalid model." (routes/personas.ts), "invalid JSON metadata" (release-metadata.ts), "migration evaluation failed" (release-info.ts:211), "Failed to substitute variables." (routes/agents/terminal-routes.ts), "Database probe failed" (observability/service-resources.ts), "Prompt delivery failed." (routes/browser-extension.ts, which also truncates via .slice(0, 2_000)).
• Different result type — release-tarball-cache.ts:210 and server/mcp-handlers.ts:577 rethrow an Error, not a string.
• Not a to-string at all — config.ts:165 (a startsWith guard), db/seed/run.ts:41 (fallback is the raw value, not String(...)).

Web-side duplicates (apps/web/src/components/app/jobs-helpers.tsx:73 reimplements the helper, components/ui/markdown.tsx:211 inlines it) are not in this PR — that half needs a re-export decision through @/lib/... and stays on the tech-debt backlog for a future run.

Behavior

None. errorMessage() is byte-identical to what each replaced site inlined.

Validation

• pnpm run check — green
• pnpm run test — green (2503 server + 705 web + 60, 0 failures)
• pnpm run test:e2e — 178 passed, 12 skipped
• finalize:web not run: no apps/web/ files changed

Queued for the next run

Backlog top item after this: the web half of this same sweep, then the formatDate name-collision cleanup across activity-chart-utils.ts / release-utils.ts / jobs-helpers.ts.

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