Support modern Apple Mail V10 directory layout

[wesm]

Summary

• Modern Apple Mail (macOS 13+) uses <GUID>/Data/Messages/ inside .mbox directories instead of a direct Messages/ subdirectory, causing import-emlx to find 0 mailboxes
• Add findMessagesDir() to probe both legacy and V10 layouts, and MsgDir field to Mailbox so the importer reads from the correct path
• Filter UUID path components from labels so account GUIDs don't leak into mailbox names

Test plan

• All existing discovery and importer tests pass (no regressions)
• New tests cover V10 walk, V10 single-mailbox auto-detect, V10 partial-emlx skipping, UUID filtering in labels, and isUUID edge cases
• Manual test with real ~/Library/Mail/V10 directory

Fixes #157

🤖 Generated with Claude Code

[roborev-ci]

roborev: Combined Review (63eeaee)

Summary Verdict: All reviewers agree that the code is clean and no issues were found.

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Synthesized from 4 reviews (agents: codex, gemini | types: default, security)

[roborev-ci]

roborev: Combined Review (6101c21)

Summary: One medium-severity issue was identified regarding Apple Mail V10 mailbox discovery prioritization.

Medium

File: internal/emlx/discover.go:198

• Issue: findMessagesDir returns the first candidate that has at least one .emlx . If both legacy Messages/ and V10 <GUID>/Data/Messages/ are populated, legacy wins by order, which can silently import a partial/stale subset and miss newer messages in V10.
• Suggested Fix: Choose the candidate with the highest non-partial . emlx count, or explicitly prefer V10 when both candidates are non-empty.

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Synthesized from 4 reviews (agents: codex, gemini | types: default, security)

[wesm]

"Not valid. In practice, a .mbox directory has either a legacy Messages/ or a V10 /Data/Messages/
layout — never both populated simultaneously. Apple Mail doesn't write messages to two different internal
layouts for the same mailbox. The only realistic mixed scenario is an empty stale Messages/ alongside a
populated V10 path, which the previous commit already handles.

Adding a "highest count" heuristic for a scenario that doesn't occur in real Apple Mail data would be
speculative complexity."