fix(core): add type guards for unknown frontmatter values in URIConstructionService

[kitelev]

Summary

• Fix 2 TypeScript errors in URIConstructionService.ts that broke CI on main since PR fix: replace console.* with ILogger in 3 core services (#2370) #2376
• frontmatter is Record<string, unknown>, but values were used as string without type guards
• Added typeof checks instead of as string assertions for runtime safety

Root Cause

PR #2376 replaced console.* with ILogger — the TypeScript errors in this file went unnoticed because unit tests pass (they don't run tsc), but build and typecheck CI jobs fail.

Impact: All 14+ commits after PR #2376 had failing CI → Auto Release skipped every time → no releases since v15.0.1.

Changes

• Line 70: Added typeof isDefinedBy !== "string" guard before passing to extractWikiLink()
• Line 97: Replaced uid || null with typeof uid === "string" ? uid : null for correct narrowing

Test plan

• tsc --noEmit --skipLibCheck passes (0 errors)
• All 212 plugin unit test suites pass
• All 69/70 CLI test suites pass (1 pre-existing integration failure)
• Pre-commit hooks pass (lint, BDD 100%, archgate)