refactor(fish): move plugin to fish/ subdirectory

Amolith created 

Prepares for multi-shell support by isolating Fish-specific files.

Fundle users should add --path 'fish' to their plugin declaration.

Assisted-by: Claude Opus 4.5 via Crush

Change summary

fish/completions/synu.fish                | 0 
fish/functions/_synu_agents/aider.fish    | 0 
fish/functions/_synu_agents/claude.fish   | 0 
fish/functions/_synu_agents/llxprt.fish   | 0 
fish/functions/_synu_agents/opencode.fish | 0 
fish/functions/_synu_agents/qwen.fish     | 0 
fish/functions/_synu_cache.fish           | 0 
fish/functions/_synu_get_quota.fish       | 0 
fish/functions/synu.fish                  | 0 
9 files changed

Detailed changes