Support finding existing signals by source URL or source label

[evkw]

Problem

When encountering a GitLab MR, Jira ticket, or other link, I need to know if Hermes already has a signal for it. Currently, sources are not guaranteed to be discoverable via search.

Goal

Allow signals to be found via source URL or label.

Scope

• Extend search to include SignalSource
• Support exact and partial matching

Acceptance Criteria

• Searching exact source URL returns the signal
• Searching partial URL returns relevant signals
• Searching source label returns relevant signals
• Multiple matches are supported
• Results clearly indicate matched source
• Works for manually created sources

UX Expectations

• Search result shows:
  • Signal title
  • Matched source label
  • Source type

Implementation Notes

• Include source fields in search query:
  • url
  • normalizedUrl
  • label
• Prefer exact match when URL is detected
• Add light URL normalization:
  • trim trailing slash
  • lowercase hostname
• Do not over-normalize query params yet

Out of Scope

• Deduplication
• Auto-linking
• Provider-specific parsing