neo MCP: `act` should let the caller suppress or cap the post-action diff (`diff: "none" | "summary" | maxChars`)

What I did: act kind=fill on a search box of a large page (a bank-reconciliation screen with ~50 rows, each rendering its own form).

Expected: a short confirmation ("ok (fill)"), or a diff only when asked for.

Actual: the post-action accessibility diff is unconditional — this one was ~14,000 tokens, ~5,000 inline and the rest spilled to ~/.browseros/tool-output/diff-<stamp>-<uuid>.md. On a large page every act costs a full page dump, most of which the calling agent cannot read (the #2671 shape, one tool over).

Suggested fix: a diff param on act (default unchanged), mirroring the maxChars PR #2674 added to evaluate — likely the act handler's settle-diff step in packages/browseros-agent/packages/browser-mcp/.

Environment: BrowserOS neo 0.49.5.0 · Chromium 148.0.7988.97 (arm64) · macOS · client = Claude over MCP.