Identify and implement code reduction opportunities

Identify top 5 opportunities to massively reduce the amount of code (lines of code) in any part of the repo: - Similar functionality that can be unified and reused - Code that has been valuable but no more - A front end component or a group of them duplicate another component/page functionality. Use the most modern and nice looking - etc. Now implement them :)

Implemented the top 5 code reductions: trimmed unreachable vendored SDK subtrees (−17.3K), deleted the quarantined PR review lane (+ fencing migration 202610251237), dead governance rules, selftest/pytest twins, dead symbols + 2 orphaned routes. Net −20,304 lines. Quick gate, SDK gate and touched API suites green; review lane pass. PR #6861 queued to the merge lane. Deferred: un-tracking the generated examples tree (owner call), v1 Agent SDK, D5 dual spellings, schema contractions. No UI surface affected.