Issue #3586298: Fix context prefix default alignment and stale getManageUrl docs.

Pravesh Pooniarequested to merge
issue/ai_context-3586298:fix/context-prefix-and-manage-docs-3586298 into 1.0.x

Fixes two RC1-blocking items from the AI Best Practices review:

  1. Context prefix default alignment: The fallback string in AiContextSystemPromptSubscriber did not match the value shipped in config/install/ai_context.settings.yml. Aligned to the canonical config default.

  2. Stale getManageUrl() docs: scope_api.md and custom_scopes.md still referenced the nonexistent getManageUrl(): ?string method. Updated to document the actual getManageRoute(): ?array method with the correct return type and structure.

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