fix(gbrain): constrain imports to configured vault

[levineam]

Motivation

• The curated import was allowing absolute paths and relative traversal to escape the configured vault, letting a manifest stage arbitrary readable files into generated GBrain pages.
• Symlinked files inside the vault could resolve to targets outside the vault, creating an additional path-traversal vector.
• The intent is to ensure imports only include files that actually live under the configured vaultDir to prevent accidental or malicious exfiltration into the brain store.

Description

• resolveSourcePath now resolves candidate source paths against the configured vaultDir and rejects paths that escape the vault (rejects .. traversal and absolute escapes).
• Added resolveRealSourcePath which canonicalizes both the vault and the candidate with realpathSync to detect and reject symlink escapes, and createImportPlan uses this canonical path for subsequent operations.
• createImportPlan now emits clearer warnings for missing/out-of-vault sources and excludes any items whose normalized or real path is outside the vault.
• Added regression tests to modules/jarvos-gbrain/test/gbrain.test.js that assert rejection of ../ traversal, absolute outside paths, and symlink-based escapes while preserving normal import behavior.

Testing

• Ran the module test suite with npm test (which executes node --test test/*.test.js) and all tests passed (36/36).
• The newly added tests exercise relative traversal and symlink escape cases and succeeded under the updated validation logic.

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Codex Task