design: TOML filter DSL specification for community-contributed command filters

[dean0x]

Design document specifying:

• TOML filter file format
• How TOML filters integrate with the OutputParser trait from B0-1
• Priority chain: built-in Rust parser > user TOML > shipped TOML > passthrough
• Trust model for project-local filters
• skim verify command for testing filter definitions
• Inline test case format

Does NOT implement the DSL — just the design doc that Phase C will implement.

Size: S
Depends on: B4
Blocks: nothing