chore: rebrand package metadata, pin engines and drop tooling cache

- package.json: homepage/repository point at this repository, node >=22 declared, @mariozechner/pi-coding-agent added as devDependency (extensions/piagentui-server.ts imports it)
- Cargo.toml: repository field filled
- .pi-lens/ ignored and its cached review graph removed from tracking
- three material icons replaced by their 32x32 sources (the committed 16x16 files were broken)

public/material-icons/folder-database.svg

@@ -1 +1 @@
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