test: test_rebuild_profiles depends on live ~/.claude data — 3 failures reproduce on main

[cdeust]

Dated remediation item (2026-07-25, §13) surfaced during #169: three test_rebuild_profiles cases fail identically on main with a bridge_finder ValueError sourced from the developer's live ~/.claude session data. Tests reading the developer's real home directory violate test isolation (results depend on whose machine runs them) — the fixture should be a synthetic projects dir. Classify per test-engineer rules: hidden shared state, not flakiness. Fix: hermetic fixture + regression test for the bridge_finder ValueError itself (it may be a real bug against unusual session data, not just a test problem).

[cdeust]

Follow-up merged: PR #183 (cold_start hermetic rewrite). Final state on the developer machine with live ~/.claude data: 5352 passed, 0 failed (EXITCODE=0, 44m33s) — the first fully green suite of this arc. The I2 ordering failure was adjudicated an environmental confound (a concurrent agent mutating the shared checkout mid-scan; proven empirically via isolated-worktree runs) — no code change warranted, the invariant scan stays strict by design. Test-isolation defect class in this repo: closed.