Add Rustdoc Coverage and Enforce Missing Docs

[brdv]

Summary

Add crate, module, and public API Rustdoc across the stck implementation and enable a missing-docs lint at the crate root.

This documents the stack planning, subprocess, and resumable-state contracts now that the module layout has stabilized, and keeps future public surface changes honest by having the compiler enforce documentation coverage.

Changelist

• src/lib.rs adds crate-level Rustdoc and enables missing_docs
• src/main.rs, src/cli.rs, src/commands.rs, src/env.rs, and src/util.rs now document their module responsibilities
• src/github.rs, src/gitops.rs, src/stack.rs, and src/sync_state.rs now document the key public types, fields, and helper contracts

Stack parent: none
Stack child: none yet

Checklist

• Changes are minimal and focused for this milestone
• No breaking CLI changes unless explicitly intended
• Errors are user-facing, actionable, and prefixed with error:
• Added/updated tests for behavior changes
• Updated docs/plan if behavior or scope changed