* fix: prototype-pollution guard in _deepMergeConfig (audit M4)
The root↔workstream config merge iterated Object.keys(overlay) with no
__proto__/constructor/prototype guard, while four sibling paths in the same
file (lines ~315/319/331/341/549) guard them. A workstream/root config.json
with {"__proto__": {...}} could pollute the merged object's prototype chain
and spoof unset config flags (per-object, not global Object.prototype).
Adds the same three-key continue guard at the top of the overlay loop plus a
regression test for __proto__/constructor/prototype overlay keys.
Closes a gap missed by the closed config proto-pollution hardening
(#751/#1406/#663).
Claude-Session: https://claude.ai/code/session_01R88n7Q54bAaVHFkDbbH1yz
* chore(changeset): Fixed fragment for #1534 (config proto-pollution guard)
Claude-Session: https://claude.ai/code/session_01R88n7Q54bAaVHFkDbbH1yz
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Co-authored-by: Tom Boucher <trekkie@nomorestars.com>
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