fix: skip symlinked Antigravity agent sources

[Success6666]

Summary

• skip symlinked workspace and skill files when building Antigravity inline sources
• resolve workspace/skills roots and candidate files before reading content so inline sources stay within the agent directory
• add regression coverage for both AntigravityTools and AntigravityAgent agent_directory loading

Fixes #8632

Tests

• python -m pytest libs/agno/tests/unit/tools/test_antigravity.py libs/agno/tests/unit/agents/test_antigravity.py
• python -m ruff check libs/agno/agno/tools/antigravity.py libs/agno/agno/agents/antigravity/agent.py libs/agno/tests/unit/tools/test_antigravity.py libs/agno/tests/unit/agents/test_antigravity.py
• python -m ruff format --check libs/agno/agno/tools/antigravity.py libs/agno/agno/agents/antigravity/agent.py libs/agno/tests/unit/tools/test_antigravity.py libs/agno/tests/unit/agents/test_antigravity.py

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