Enforce strict compiler checks and declaration groups

[jdeurt]

The compiler previously checked nullable values but left other strict checks and JavaScript diagnostics off. Enable strict, checkJs, and the additional indexed-access, optional-property, control-flow, unused-code, module, and declaration-file checks. Keep every owned JavaScript and TypeScript source in the program, and fix the exposed errors with checked values and schemas.

Update jdeurt/const-on-top to reject const–let–const and let–const–let declaration sequences. Either kind can come first. Preserve declaration prefixes, ambient bindings, loop headers, and independent nested scopes. Order checks do not depend on whitespace, and the rule does not reorder code. The existing spacing rule remains separate.

Disable only unicorn/no-null. Compiler null safety remains enabled. Update the shared skill and maintenance documentation to match these policies.

Validation:

• bun run lint, bun run check-types, bun run format:check, bun run imports:check, bun run build, bun run check, bun test, and the publication dry run pass.
• All 14 existing tests pass. The change adds no permanent test cases.
• 76 temporary declaration and whitespace cases pass without autofixes. All 35 owned source files enter the compiler program. JavaScript, indexed-access, and null diagnostics remain active.
• Both client packages contain identical root policies and style guidance. The build contains five plugins and 608 generated files.
• Independent code and skill review found no actionable issues. Clone and dead-code review found no new issue requiring a change.