fix: parse wiki shorthand targets

[zhangyangyu]

Description

Aligns backend wiki shorthand handling so double-bracket links treat the left side as the page target and preserve the label separately. This updates catalog extraction, backlink matching, and move rewriting for the target-before-label form while keeping single-argument shorthand behavior intact. Ordinary Markdown links keep their existing behavior.

Tracking

N/A

Tests

• make check
• go test ./internal/wikicatalog ./internal/service ./internal/rest

SSOT Drift Prevention Checklist

• Checked that this PR does not violate boundaries in docs/architecture.md or docs/module-contracts.md
• Package structure and dependencies are unchanged; no SSOT doc update needed
• Test conventions and coverage expectations are unchanged; no docs/test-strategy.md update needed
• CI, smoke, regression, and packaging workflows are unchanged; no docs/ci.md update needed