Fix dev setup: gitignore, formatting, and schema build rules

[lykimq]

• Add missing files to .gitignore (credentials, generated files, editor artifacts)
• Reformat OCaml source files via dune build @fmt --auto-promote
• Change schema fetch rules (github-schema.json, gitlab-schema.json) from mode promote to mode fallback so dune build no longer triggers network fetches on every run; add named aliases (update-github-schema, update-gitlab-schema) for explicit schema refresh.

[Zimmi48]

I agree with changing the default to not re-fetch the schema every time, but:

• the doc should be updated (currently it says to use dune build --ignore-promoted-files for this reason, there's even one use in CI)
• I tested the alias and it doesn't seem to work to regenerate the files without the universe dependency (unless I am missing an option, which should be documented).

BTW, it would be very convenient to have CI jobs to:

• auto-check formatting so that we don't have to fix it anymore
• open PRs and auto-merge them if CI passes to update the GraphQL schemas (BTW, I just saw that updating the GitHub GraphQL schema breaks the build, but I didn't investigate further yet).

[lykimq]

• the doc should be updated (currently it says to use dune build --ignore-promoted-files for this reason, there's even one use in CI) 94e73ac and 6026f35
• I tested the alias and it doesn't seem to work to regenerate the files without the universe dependency (unless I am missing an option, which should be documented).
  I have fixed and update it in the commit 8219a77

[lykimq]

BTW, it would be very convenient to have CI jobs to:

auto-check formatting so that we don't have to fix it anymore
open PRs and auto-merge them if CI passes to update the GraphQL schemas (BTW, I just saw that updating the GitHub GraphQL schema breaks the build, but I didn't investigate further yet).

For those I will open another PRs.