Centralize link URL validation

docs/internal/link.md

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+# Link URL Policy
+
+## Purpose
+
+`internal/link` defines shared validation for configured and persisted HTTP link URLs.
+
+## Inputs and outputs
+
+Input is a URL string. Output is either nil for an accepted URL or a concise validation error that callers wrap with field context.
+
+## Validation behavior
+
+Accepted URLs must parse successfully, use `http` or `https`, include a host, and omit query strings and fragments.
+
+## Boundaries
+
+This package validates URL shape only. It does not construct destination output URLs, choose primary URLs, infer public URLs from backend configuration, or read configuration files.
+
+## Tests
+
+Before changing link URL policy, inspect tests under `internal/link` and callers in `internal/config`, `internal/state`, and `internal/publish`.
+
+## Invariants
+
+- Configured `links.base_url`, persisted `links.primary_url`, persisted output `url`, and publish link planning use the same URL policy.
+- Callers own field-specific error context.
+- URL path construction remains in `internal/publish`.