pi: add agent config, themes, subagents, and extensions

.pi/agent/extensions/lsp-feedback/README.md

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+# pi lsp-feedback
+
+Auto-detected defaults for this machine:
+
+- `.nix` -> `~/.nix-profile/bin/nixd` + `alejandra`
+- `.sh`, `.bash` -> `~/.nix-profile/bin/bash-language-server start` + `shfmt -w`
+- `.lua` -> `~/.nix-profile/bin/lua-language-server` + `stylua`
+
+## What it does
+
+- Overrides pi `write` and `edit`
+- After a file change, optionally formats the file
+- Re-syncs the final file content into the language server
+- Adds a compact diagnostics summary to tool output
+- Shows a compact footer status only after supported LSP activity
+
+## Commands
+
+- `/diag path/to/file` - compact diagnostics for one file
+- `/lsp-restart` - restart managed language servers
+
+## Tool
+
+- `lsp_diagnostics` - on-demand diagnostics for supported files
+
+## Notes
+
+- This MVP is intentionally compact and context-efficient.
+- It stays silent on startup and only becomes visible after you touch or check a supported file.
+- Whole-project diagnostics are not implemented yet.
+- If you change this extension, run `/reload` in pi.