fix: schema write from stdin

[miparnisari]

Addresses #531

Before

[16:24:26] ~/Documents/GitHub/zed (write-schema-tests) $ cat ~/Desktop/basicschema.zed|  go run ./cmd/zed/main.go schema write
4:24PM ERR terminated with errors error="must provide file path or contents via stdin"
exit status 1

After

[16:24:53] ~/Documents/GitHub/zed (write-schema-tests) $ cat ~/Desktop/basicschema.zed|  go run ./cmd/zed/main.go schema write
4:26PM WRN not calling a released version of SpiceDB version=v1.45.5-0.20250923215517-7491a826fc13+enterprise.devel.(devel)

[tstirrat15]

Coupla WIP comments

[codecov-commenter]

Codecov Report

❌ Patch coverage is 31.81818% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 40.06%. Comparing base (7ed5ab0) to head (7a65897).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/cmd/schema.go	33.33%	13 Missing and 1 partial ⚠️
internal/commands/schema.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #557      +/-   ##
==========================================
+ Coverage   39.28%   40.06%   +0.78%     
==========================================
  Files          37       37              
  Lines        5448     5473      +25     
==========================================
+ Hits         2140     2193      +53     
+ Misses       3063     3029      -34     
- Partials      245      251       +6     

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[tstirrat15]

LGTM