fix(harness): deny WebSearch/WebFetch on read-only claude workers

[solomonneas]

Summary

Fixes #456. A read-only Claude worker could stall with ok=false, exit_code=5, "Claude is waiting for a permission prompt" when Claude Code requested approval for a Web Search tool call. --permission-mode plan fails closed on writes, but the built-in web tools (WebSearch, WebFetch) still route to an interactive approval prompt that a non-interactive Brigade worker cannot answer.

Root-cause fix at the adapter boundary (sibling of #446): add WebSearch,WebFetch to _CLAUDE_DISALLOWED_READ_ONLY in src/brigade/agents.py, hiding the web tools from the read-only tool surface exactly like Bash/Edit/Write. A read-only REVIEW worker has no need for web tools.

• Deny list stays a single comma-joined argv element; the -- end-of-options marker between it and the prompt is untouched (no bug(harness): Claude read-only prompt is consumed as --disallowedTools values #446/fix(harness): keep claude prompt out of variadic --disallowedTools #451 variadic regression).
• The danger-full-access branch uses --dangerously-skip-permissions, so no prompt can block there; its deny list (Task,Agent) is unchanged.

Tests

• New test_claude_read_only_denies_web_tools_that_prompt_for_approval: asserts on the built argv that WebSearch/WebFetch are in the read-only deny list, the prompt is still separated by -- and intact as the final positional, and the danger-full-access branch is unchanged.
• Updated the three existing tests that pinned the old deny-list literal.

Verification

• brigade work verify run --target . --command "pytest -q tests/test_agents.py" --capture brigade-work → exit=0
• brigade work verify run --target . --command "./scripts/verify" --capture brigade-work → exit=0 (ruff, format, version-sync, mypy, full pytest + coverage floor)

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