Switch launcher config from TOML+serde+liquid to Starlark AST parsing.
The new format uses function-call syntax (version(), local(), github(),
http()) that is statically analyzable without full evaluation.

- Remove liquid and toml dependencies, add starlark_syntax
- Parse version.axl via AST walking instead of serde deserialization
- Use str::replace for http URL templates instead of liquid
- Derive github release/artifact defaults at runtime when omitted

Use positional version arg, rename release to tag, simplify template vars

- version() now takes version as first positional arg instead of use=
- github() uses tag instead of release field name
- Template variables simplified to {version}, {os}, {arch}, {target}
  - {os} = kernel name (darwin, linux) from uname -s
  - {arch} = ISA name (x86_64, aarch64) from uname -m
  - {target} = GNU triple (aarch64-apple-darwin)
- Extract replace_vars() helper for consistent replacement
- Apply template replacement to github tag/artifact fields too

Create version.md