fix: guard-scan MCP inputSchema strings and cap sizes (M-4)

[jkyberneees]

Closes the MCP inputSchema tool-poisoning channel (M-4).

Changes:

• cmd/odek/mcp_approval.go: recursively guard-scans every string in an MCP tool's inputSchema; rejects schemas larger than 256 KiB; shows a SHA-256 hash + byte size in the approval prompt.
• cmd/odek/main.go: passes the guard instance and config to approveMCPTools.
• Added regression tests for schema hash display, tainted-schema skipping, and oversized-schema skipping.
• Updated docs/SECURITY.md and AGENTS.md.