feat: add permission case system with required context for write/patc…

[jmikedupont2]

…h operations

• Add required context field to FSWrite, FSPatch, FSMultiPatch tool calls LLM must provide a justification for every write/patch/multipatch operation
• Introduce PermissionCase struct for judicial-style evidence collection
• Show proposed content/diff before TUI permission prompt
• Include caller context (explanation) in the case panel
• Update tool descriptions JSON schema with context field
• Fix nixpkgs lock for required rustc 1.94+
• Update all snapshot tests for new schema

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