Sweep of every consumer that previously depended on either the
standalone [nox-xdge] package (the Eio wrapper) or dune's internal
[xdg] library. Both are gone after the previous commit; consumers
now point at the merged [nox-xdg] package's two libraries:

- Code that used the Eio wrapper ([Xdge.v fs app_name],
[Xdge.config_dir], etc.) -> [Xdg_eio.foo], library [nox-xdg.eio].
- Code that used the pure spec ([Xdg.create ~env ()],
[Xdg.cache_dir]) -> unchanged module name [Xdg], library
[nox-xdg].

Touched packages:

- gauth, gdocs, gsheets, gslides -- the Google credential stack;
[Gauth.Local_store] uses [Xdg_eio] internally.
- ocaml-requests, ocaml-cookie -- HTTP client and cookie jar;
[Requests.t] takes an [?xdg:Xdg_eio.t] for cookie persistence.
- ocaml-agent, ocaml-atp, ocaml-slack, ocaml-linkedin, monopam,
uniboot -- CLIs that compute their own config / cache / state
paths via the Eio wrapper.
- ocaml-oci -- uses the pure spec ([Xdg.cache_dir]) for OCI image
layout; migrated from dune-internal [xdg] to [nox-xdg].

Mechanical changes per package:

- [dune-project]: opam dep [nox-xdge] -> [nox-xdg].
- [lib/dune] and [bin/dune]: library reference [nox-xdge] ->
[nox-xdg.eio] (or [nox-xdg] for the rare pure-spec consumer).
- [.ml] / [.mli]: [Xdge] -> [Xdg_eio].

Generated [.opam] files updated by the dune build accordingly.

Remove libraries declared in '(libraries ...)' clauses but unreferenced
by any module in the same source tree, as flagged by 'monopam lint'
after the new Dead_lib detection landed. Touches 131 dune files across
~80 packages.

A few stanzas needed a positive correction instead of a pure removal:

- ocaml-git/bin/diag: depended on eio_main + bytesrw-eio for an
Eio_posix.run call site; the umbrella was overkill, switch to the
precise eio_posix package.
- ocaml-scaleway/lib, ocaml-s3/lib: scaleway.mli / s3.mli reference
Eio_unix.Stdenv.base; eio.unix is required and was missing.
- merlint/lib: pulled bytesrw + nox-opam.bytesrw to surface
Opam_bytesrw, used by rule e915 and lint helpers.

Stanzas where Dead_lib was a false positive (transitive dep needed
for module visibility, virtual-library impls) are left untouched —
e.g. helix.jx.jsoo for ocaml-globe/demo retains its (libraries ...)
entry because it provides the impl of the helix.jx virtual lib.

The READMEs all share the standard install/overlay snippet, but the
sh blocks lacked the "<!-- $MDX skip -->" directive. `dune test`
would shell out to `opam install` against the live switch, which
either prompts interactively or fails with a package conflict —
either way diffing as a test failure.

Bulk-add skip directives in front of every install/overlay block.
Also collapse the doubled "non-deterministic + skip" stack on three
READMEs (memtrace, ocaml-dpop, ocaml-pid1, ocaml-yaml, merlint) where
`skip` already implies the runtime is bypassed.

Extends the nox- prefix to the remaining encoding/codec packages —
none clash with opam-repository today, but the rule "blacksun forks
get nox-" applies the same way regardless of conflict status.

Renamed: json, xml, meta, opam, protobuf -> nox-*

Renames 35 packages to make blacksun forks distinguishable from their
opam-repository upstreams. Module names (Git.x, Tls.x, ...) stay bare;
opam package names and dune (public_name) findlib references move to
nox-X. After this commit, zero local package names overlap with
opam-repository.

Renamed:
- nox-git, nox-irmin
- nox-crypto, nox-crypto-pk, nox-crypto-rng, nox-crypto-ec
- nox-tls, nox-tls-eio, nox-tar, nox-tar-eio, nox-tty, nox-tty-eio
- nox-arp, nox-ca-certs, nox-cbor, nox-cookie, nox-crc, nox-csv
- nox-gpt, nox-hkdf, nox-http, nox-jwt, nox-kdf, nox-loc
- nox-memtrace, nox-pds, nox-sexp, nox-slack, nox-toml
- nox-websocket, nox-x509, nox-xdge, nox-yaml

Also drops orphan tar-mirage and tar-unix opam templates that had no
matching package stanza.