DEVX-164: auto-merge resilience — self-approval and Vikunja outage handling

Problem

Two defects hit auto-merge during S02 work:

get_vikunja_task_title crashes on transient Vikunja errors. The Vikunja API returned 404/502 during a restart window (run 6093); list_project_tasks treats 4xx as non-retryable APIError, so the job failed immediately instead of riding out a short outage.
When a PR author and the workflow's reviewer token map to the same Gitea user, the auto-approve step is rejected ("approve your own pull is not allowed") and the merge fails 405: not enough approvals. The generic merge error gives no hint that an external approval is the fix (hit on sso-bridge #18, #19, and infra #1647's approvals-only failure mode).

Approach

REQ-1: Wrap the task-list pagination in get_vikunja_task_title with a bounded retry (tenacity, ~4 attempts, exponential backoff) covering APIError and requests.RequestException. A genuinely missing task still ends in the same "Could not find" ClickException.

REQ-2: On merge HTTP 405, fetch PR reviews; when zero APPROVED reviews exist, extend the error with the self-approval explanation and the remediation (approve via a non-author account).

REQ-3: Regression tests for both behaviors.

Files Affected

src/devx/ci/auto_merge.py
tests/unit/test_auto_merge.py

Test Plan

New tests: retry-then-success on transient APIError; retry-exhaustion still raises; missing task still raises; 405 error includes approvals diagnostic.
make pytest-cov, make lint-all.

Deploy Plan

Merge → next release publishes the package; consuming repos pick it up on their next CI run (devx is pinned per-repo, bump via the usual dependency PR flow).

Rollback Plan

Revert; previous behavior returns.

Acceptance Criteria

REQ-1: Vikunja task-list retries transient API failures
REQ-2: 405 merge error reports approval state + self-approval hint
REQ-3: Regression tests added and passing

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