fix(security): harden path boundaries and sanitize mcp inputs

[sknr]

Replaced filepath.ToSlash with strings.ReplaceAll for Windows path normalization in resolveInBundle and resolveBundleDir to fix path traversal boundaries on POSIX systems. Added getStringArg helper in mcp.go to enforce validation, type-checking, and bounds limitations (1KB constraints, 1MB body limit) on strings for MCP tool inputs. Added negative tests to mcp_test.go and mutate_security_test.go to verify these boundary constraints and malicious inputs rejection.

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PR created automatically by Jules for task 16583702038588133826 started by @sknr

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[sknr]

Superseded and completed on develop via commits 809f521, 41682b4, 05b6fc5, a7d0bbe, 1647508, and b3758c5.

[google-labs-jules]

Superseded and completed on develop via commits 809f521, 41682b4, 05b6fc5, a7d0bbe, 1647508, and b3758c5.

Understood. Acknowledging that this work is now obsolete and stopping work on this task.