Clarify and regression-test explicit stdin body handling

[FelixLisczyk]

Problem

Before this change, command documentation implied that piping input was sufficient:

printf'Description'| linear issues create "Issue title"

However, the CLI only consumed stdin when the corresponding flag explicitly had the value -. The piped content could therefore be silently ignored, producing an empty description or leaving an existing description unchanged. The same documentation inconsistency affected project descriptions and issue comment/reply bodies.

Automatic stdin detection is intentionally not restored because it can cause commands to block in automation environments.

Changes

This PR makes the existing explicit-stdin contract consistent and clearly documented:

printf'Description'| linear issues create "Issue title" --description -
printf'Comment body'| linear issues comment ISSUE-123 --body -

It also:

• Adds an injectable stdin-reading seam for deterministic tests.
• Adds propagation tests for issue create/update, project create/update, comments, and replies.
• Covers whitespace trimming, empty input, read errors, and non-consuming non-explicit input.
• Updates command help, examples, README guidance, onboarding text, and embedded skill documentation.
• Preserves ordinary flag values, attachment handling, and existing update semantics.

Testing

• make test passes.