Browser: allow config default for efficient snapshots

[sebslight]

Summary

• add browser.snapshotDefaults.mode (currently supports "efficient") to the config schema/types
• apply the config default in the browser tool + browser CLI snapshot command when no mode is supplied
• add tests for the new default behavior and ARIA guard
• document the new config knob in gateway config and browser tool docs

Why

We want a configuration switch that makes efficient snapshots the default only when explicitly configured, so operators can reduce snapshot size/noise without forcing all callers to pass mode=efficient in every tool call or CLI invocation.

Why this approach

• Keep the setting under browser.* because it controls browser snapshot behavior, not agent policy or tool gating.
• Apply the default at the tool/CLI layer so it works for LLM tool calls and CLI usage without altering server semantics for other HTTP clients.
• Limit the schema to mode: "efficient" for now to minimize scope while matching existing behavior; the config does nothing unless explicitly set.

Tests

• pnpm test -- src/agents/tools/browser-tool.test.ts

[steipete]

Landed via squash on main.

Commit:

• a0cd295 fix: add browser snapshot default mode (Browser: allow config default for efficient snapshots #1336)

Tests:

• pnpm lint
• pnpm build
• pnpm test

[steipete]

Closing since changes landed on main via squash (commit a0cd295).