fix(nix): generate shell completions via Effect CLI flag - #51406

Three independent defects in the v2 Nix packaging, each fatal to
`nix build .#opencode` or `nix build .#opencode-desktop`.

`nix/opencode.nix` generates completions through `opencode completion`, a
subcommand v2 no longer has. The Effect CLI parses it as the optional
`directory` argument and the TUI opens instead, so `installShellCompletion`
gets a zero-size file and the build fails:

    Error: ENOENT: no such file or directory, chdir '/build/source/packages/cli' -> 'completion'
    ERROR: installShellCompletion: installed shell completion file .../opencode.bash does not exist or has zero size

Completions now come from the Effect-CLI global action flag,
`--completions <bash|zsh|fish|sh>`. Naming the shell directly also drops
the `SHELL=/bin/zsh` coaxing the old yargs-based path needed. `sh` is a
synonym for `bash`, so `--bash` already covers it; fish did not ship
before even though the CLI generates it.

Each shell is staged to a file before installation. A process substitution
does not propagate the generator's exit status, so a failure here would
install a zero-size completion and still pass — which is how the bug above
presented in the first place.

The command name is a build-time define, so `opencode2 --completions` just
reproduces the `opencode` scripts. They are renamed for the alias with
`substitute --replace-fail`, so a future change to the baked-in name breaks
the build instead of quietly shipping a second copy of the opencode one.

`nix/electron.nix` still carries the electron 42.10.1 hashes while
`packages/desktop/package.json` moved to 44.4.3, so the desktop's electron
fetchzip cannot resolve. The hashes are refreshed from the 44.4.3
SHASUMS256.txt; the headers hash is the fetchzip recursive hash, computed
the same way the 42.10.1 one was (reproducing the previous value as a
control). The x86_64-darwin entry is dropped: nixpkgs' electron `tags` has
no such platform and `get` throws for it, so the value was never selected.

`nix/desktop.nix` stages the CLI executable but not the package manifest
next to it, and `packages/desktop/scripts/utils.ts` reads the bundled CLI's
version from that manifest. The desktop build aborts in `scripts/prebuild.ts`:

    ENOENT: no such file or directory, open '/build/desktop-cli/cli-linux-x64-baseline/package.json'

The manifest is now written alongside the executable, so the version file
the desktop ships resolves without spawning the binary.

Fixes anomalyco#50408

Co-developed-by: @ReStranger (anomalyco#51406), @mindnbytes (anomalyco#50409)

Three independent defects in the v2 Nix packaging, each fatal to
`nix build .#opencode` or `nix build .#opencode-desktop`.

`nix/opencode.nix` generates completions through `opencode completion`, a
subcommand v2 no longer has. The Effect CLI parses it as the optional
`directory` argument and the TUI opens instead, so `installShellCompletion`
gets a zero-size file and the build fails:

    Error: ENOENT: no such file or directory, chdir '/build/source/packages/cli' -> 'completion'
    ERROR: installShellCompletion: installed shell completion file .../opencode.bash does not exist or has zero size

Completions now come from the Effect-CLI global action flag,
`--completions <bash|zsh|fish|sh>`. Naming the shell directly also drops
the `SHELL=/bin/zsh` coaxing the old yargs-based path needed. `sh` is a
synonym for `bash`, so `--bash` already covers it; fish did not ship
before even though the CLI generates it.

Each shell is staged to a file before installation. A process substitution
does not propagate the generator's exit status, so a failure here would
install a zero-size completion and still pass — which is how the bug above
presented in the first place.

The command name is a build-time define, so `opencode2 --completions` just
reproduces the `opencode` scripts. They are renamed for the alias with
`substitute --replace-fail`, so a future change to the baked-in name breaks
the build instead of quietly shipping a second copy of the opencode one.

`nix/electron.nix` still carries the electron 42.10.1 hashes while
`packages/desktop/package.json` moved to 44.4.3, so the desktop's electron
fetchzip cannot resolve. The hashes are refreshed from the 44.4.3
SHASUMS256.txt; the headers hash is the fetchzip recursive hash, computed
the same way the 42.10.1 one was (reproducing the previous value as a
control). The x86_64-darwin entry is dropped: nixpkgs' electron `tags` has
no such platform and `get` throws for it, so the value was never selected.

`nix/desktop.nix` stages the CLI executable but not the package manifest
next to it, and `packages/desktop/scripts/utils.ts` reads the bundled CLI's
version from that manifest. The desktop build aborts in `scripts/prebuild.ts`:

    ENOENT: no such file or directory, open '/build/desktop-cli/cli-linux-x64-baseline/package.json'

The manifest is now written alongside the executable, so the version file
the desktop ships resolves without spawning the binary.

Fixes anomalyco#50408

Co-developed-by: @ReStranger (anomalyco#51406), @mindnbytes (anomalyco#50409)

Three independent defects in the v2 Nix packaging, each fatal to
`nix build .#opencode` or `nix build .#opencode-desktop`.

`nix/opencode.nix` generates completions through `opencode completion`, which
v2 removed: the Effect CLI parses it as the optional `directory` argument, so
the TUI opens instead and `installShellCompletion` gets a zero-size file. It
now uses the global `--completions <shell>` flag, stages each shell to a file
so a generator failure fails the build, and renames the `opencode2` copies
because the command name is a build-time define. Fish ships now; `sh` is a
synonym for `bash`.

`nix/electron.nix` carries the electron 42.10.1 hashes while
`packages/desktop/package.json` moved to 44.4.3, so the desktop's electron
fetchzip cannot resolve. Refreshed from the 44.4.3 SHASUMS256.txt; the
headers hash recomputed the same way the 42.10.1 one was, reproducing it as
a control. The x86_64-darwin entry is dropped, nixpkgs' electron tags have
no such platform.

`nix/desktop.nix` stages the CLI executable but not the manifest that
`packages/desktop/scripts/utils.ts` reads the version from, so
`scripts/prebuild.ts` aborts on a missing package.json.

Fixes anomalyco#50408

Co-developed-by: @ReStranger (anomalyco#51406), @mindnbytes (anomalyco#50409)