fix(security): prevent search_code symlink path escape

[study8677]

Motivation

• search_code walked workspace paths and read files without resolving targets or enforcing is_safe_path, allowing in-repo symlinks to disclose files outside the project.
• The structured-agent tooling exposes these tool outputs to LLMs and persists raw outputs, increasing the severity of any disclosure.

Description

• Resolve each candidate path in search_code and skip any target where is_safe_path(ws, resolved) is false so symlink targets outside the workspace are not read.
• Read file contents from the resolved, validated path (resolved.read_text(...)) instead of the original fpath.
• Add a regression test test_search_code_skips_symlink_outside_workspace that creates an in-repo symlink to an outside file and asserts search_code does not return matches from that symlink.
• Modified files: engine/antigravity_engine/hub/ask_tools.py and engine/tests/test_ask_tools.py.

Testing

• Ran pytest -q engine/tests/test_ask_tools.py -k "search_code", which completed successfully with 7 passed, 11 deselected.
• The new regression test verifies symlink escapes are blocked and the existing search behavior is preserved for normal files.

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Codex Task