fix: validate suggests absolute path for relative link errors

[akeemjenkins]

Problem

Relative markdown links like [Cloaked](organizations/cloaked) from a concept at pages/about.md resolve to pages/organizations/cloaked (broken). The correct form is [Cloaked](/organizations/cloaked) (absolute). The error message didn't explain this gotcha.

Fix

When validate finds a broken link, it checks if the target would resolve as an absolute path. If so, the error message suggests the absolute path: use /organizations/cloaked for an absolute path.

[akeemjenkins]

Review — reviewed against current main (d641562), test-merged locally

Verified: merges cleanly; go build / go vet / go test ./... pass, and golangci-lint is clean (0 issues) on the merged result.

The fix is correct and safe. When a relative link fails to resolve, the absolute-path suggestion is only emitted after b.HasConcept(absTarget) confirms it actually resolves, so it never proposes a path that isn't real. Already-absolute-but-broken links, external URLs, and root-level concepts (where relative and absolute coincide) all correctly avoid a spurious suggestion. Good doc comments, and both the positive (suggestion present) and negative (genuinely-missing concept must not mention "absolute path") cases are tested.

Nits (optional)

• absoluteTarget re-implements what resolveLink("", link) already produces (an empty fromConceptID gives the bundle-root interpretation). Reusing it drops ~18 lines.
• The suggestion strips any #fragment (e.g. organizations/cloaked#s → suggests /organizations/cloaked). Cosmetic — fragments are ignored for resolution anyway — but preserving it would be friendlier.
• Add a test for the already-absolute-and-broken case to lock in that it does not suggest.

For the record: an earlier note worried the test file wasn't gofmt-clean. CI does not run a gofmt/gofumpt linter (.golangci.yml enables govet, staticcheck, errcheck, ineffassign, unused, revive, gosec, misspell), and the merged branch lints clean — so that is not a blocker.

Recommendation: approve — the nits are optional follow-ups.

_{🤖 Automated review via Claude Code, verified locally against current main.}