fix(importers): Canonicalize absolute importer paths

[reneleonhardt]

What does this PR do?

Makes absolute --importers queries use the same canonical project and file roots:

• Infers the nearest Git project for an absolute importer launched outside the project.
• Canonicalizes the importer in both explicit-root and inferred-root paths.
• Preserves explicit project-root precedence.
• Adds end-to-end coverage for canonicalized reports and root precedence.

Why it matters

On macOS, /tmp and /private/tmp can name the same file. Mismatched spellings produced incorrect relative paths and empty importer results.

CLI / MCP surface

No new commands, arguments, or MCP tools. Existing --importers queries now work consistently with absolute paths.

Verification

• codemap . and codemap --deps .

The repository linter reports no new findings for this branch beyond its existing baseline.

Co-Authored-By: GPT-5.6 Sol codex@openai.com

[JordanCoin]

Verified and merged. Canonicalization holds up under the attacks that matter: cross-repo symlink aliasing, nested repo-in-repo, a submodule, and a linked worktree (.git as a file) all resolve to the file's true identity — I couldn't get it to map one file onto another's identity. Dangling symlinks and no-git-ancestor paths fail closed with bounded errors rather than a confident empty answer. Relative-path --importers output is byte-identical to main across a dozen real files, and the new tests are load-bearing (reverting the fix fails three of the four). Clean merge, suite at baseline.

One adjacent gap worth a follow-up, pre-existing and not from this PR (confirmed byte-identical on main, so nothing here regressed): in buildImportersReportFromGraph the filepath.Rel(root, file) result isn't checked for staying inside root. With an explicit root and an absolute --importers path outside it, Rel happily walks up and the tool prints No files import ../../home/user/... with exit 0 instead of erroring — a confident empty answer in exactly the shape this project treats as the cardinal sin. Guarding on strings.HasPrefix(rel, "..") || filepath.IsAbs(rel) would close it; happy to look at that separately whenever you want it picked up.

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