docs: standardize repository decisions and change records

[danielblignaut]

Contributors now use the same ADR and change-record workflow, with agent instructions, PR/pre-commit skills, templates, and a PR documentation checklist requiring records to stay current through review. Existing repository-specific guidance and decision history are preserved.

Validation: checked changed skill frontmatter, local documentation links, Markdown fences, required record sections, and git diff --check. Runtime builds, browser flows, and provider tests were not run for this documentation change.

Documentation: decision, complete update record. Related repositories can adopt this convention independently; no coordinated application deployment or secret migration is required.

The branch was reconstructed from the fetched default branch in an isolated worktree. Unrelated changes in the original checkout are excluded. Review comments and current checks were inspected before merge; no external review bot was requested.

The shared Mintlify submodule moves from docs/ to public-docs/, preserving its pinned commit. The Go generator default, Makefile, README, and provider skills change together. Existing checkouts must preserve submodule changes and run git submodule deinit docs before updating the branch, then run git submodule sync and git submodule update --init --recursive public-docs; custom scripts using the old path must be updated or set DOCS_REPO_DIR. Focused go test ./internal/docgen ./tools/generatedocs passed. A temporary initialized-submodule fixture also verified the deinit-before-update migration leaves a clean checkout and independent documentation paths.