Make the fixture guard survive concurrent worktree churn

[chubes4]

Fixes #1282.

Problem

tests/worktree-add-lifecycle-fixture-guards.php failed inside the release test runner while passing standalone, blocking homeboy release data-machine-code twice on code that was fine.

PHP Fatal error:  Uncaught RuntimeException: worktree-add-lifecycle could not inspect an inspectable managed worktree registry.
 in tests/worktree-lifecycle-fixture-guard-support.inc:75

Immediately afterward, standalone:

$ php tests/worktree-add-lifecycle-fixture-guards.php
worktree-add-lifecycle-fixture-guards ok: ...

./bin/dmc-test → 148 passed, both before and after.

Cause

The guard seeds its inspection set with the current working directory:

$repositories = array( $current_directory );   // a live repo, not a fixture

On a developer machine or CI runner that is a real repository which other processes may be concurrently adding worktrees to or removing them from — during this session, an agent-driven cleanup was removing 93 worktrees. A single git worktree list --porcelain against such a repository can fail transiently, and the guard treated any non-zero exit as fatal:

if ( 0 !== $status ) {
    throw new RuntimeException('worktree-add-lifecycle could not inspect an inspectable managed worktree registry.');
}

So the test was order- and timing-dependent on external state it does not own. The throw also discarded the captured $output, leaving no repository, exit code, or git message to act on.

Change

Extract the inspection into worktree_lifecycle_fixture_inspect_registry(), which retries a bounded number of times (3, 250ms apart) before failing closed, and reports real evidence when it does.

Fail-closed safety is unchanged — a repository that genuinely cannot be inspected still fails. Only transient churn is absorbed.

Verification

Diagnostics and retry both exercised against a non-repository path:

retried_for=1.46s
message=worktree-add-lifecycle could not inspect an inspectable managed worktree registry after 3 attempts.
        repository=/tmp exit_code=128
        git_output=fatal: not a git repository (or any of the parent directories): .git

• retries actually occur (1.46s ≈ 3 attempts × 250ms plus git startup)
• the error names the repository, exit code, and git output

./bin/dmc-test → 148 tests passed. Target test passes standalone.

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AI assistance disclosure: implemented with Claude Sonnet 4.5 (Anthropic) driving OpenCode via the Kimaki Discord bridge. The model hit this failure while releasing v0.72.8, traced it to the cwd-seeded inspection set, made the change, and captured the verification output above. A human directed the work.