feat: add stdin request support

[inickt]

Enables reading the request json from STDIN when - is specified as an arg instead of a file path. - is commonly used in CLI tools to indicate reading from STDIN.

For example, pbpaste | lk sip inbound create - to use clipboard contents (on macOS) for creating a SIP trunk.

As I write this I realize pbpaste | lk sip inbound create /dev/stdin is functionally equivalent without code changes, so if that is preferred feel free to close...

disclaimer: mostly generated by codex, with manual review

Summary by CodeRabbit

Release Notes

• New Features

  • Added support for reading input from stdin by using "-" as an argument, providing an alternative to file paths.
  • Updated help text to document stdin input capability.

• Bug Fixes

  • Added validation to prevent multiple stdin arguments in a single command.

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[CLAassistant]

All committers have signed the CLA.

[coderabbitai]

📝 Walkthrough

Walkthrough

The changes add stdin support to the request processing logic by treating "-" as a signal to read from standard input. The ReadRequestFileOrLiteral function now checks for "-" before attempting file operations, RequestDesc appends a hint about stdin capability, and createAndPrintReqs validates that "-" is used at most once across multiple arguments.

Changes

Cohort / File(s)	Summary
stdin Support Enhancement cmd/lk/proto.go	Modified ReadRequestFileOrLiteral to read from stdin when "-" is provided; updated RequestDesc with "(or - for stdin)" hint; added validation in createAndPrintReqs to prevent multiple stdin usage when processing multiple arguments.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Poem

🐰 A dash now opens stdin's door,
No files needed anymore!
Just "-" and watch the data flow,
One stream at once—that's all you need to know! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: adding stdin request support via the '-' argument convention.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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