Harden dogfooding diagnostic path redaction - #2741

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Fresh-context review record (coordinator, 2026-09-06; read-only Opus 5 reviewer over the diff against the merge base; route: the standing D-12 rule for Codex-session PRs — review, fix MEDIUM+ in-PR, merge, disposition comment).

VERDICT: SHIP (round 1). Traced by hand and sound: drive root, UNC root, sibling-prefix, case-fold only on Windows, home="/", relative paths; ntpath.abspath behaves on POSIX hosts so the Windows fixtures are host-independent. Fixed in-PR (round 2, coordinator commit d952d8598, unittest suite OK): MEDIUM-2 — the No such database exit message printed the raw --db/$TASKDECK_DOGFOOD_DB path (username, org name), the one line most likely to be pasted into an issue; it is now redacted. Recorded for #1480, not fixed: MEDIUM-1 — no CI job executes test_dogfood_snapshot.py (reusable-docs-governance.yml discovers only the agent_hooks and analysis suites); wiring it is a control-plane change left for the CI lane. LOW — a POSIX filename containing a literal backslash renders as a directory boundary (pre-existing).