Allow i report to accept piped natural-language input

[CoreyRDean]

Non-technical summary

i report can now take natural-language input from stdin as well as argv, so report drafting works in Unix-style composed flows like i report < notes.txt and echo 'more detail' | i report 'first detail'.

This matters now because the repo explicitly treats composability as a core product promise, and i report was breaking that promise by silently ignoring piped text.

After this change, the command behaves more like the rest of the tool: it accepts natural-language input from the shell surface the user gives it instead of forcing everything onto the command line.

Technical summary

• read non-TTY stdin in cmdReport using the same bounded pipe-reading helper already used by natural-language mode
• merge argv text first, then append trimmed stdin content, so the final report prompt preserves the ordering requested in issue Add support for stdin input in i report #21
• keep the existing no-input usage path intact when both argv and stdin are empty
• add focused unit coverage for args-only, stdin-only, args-plus-stdin, and blank-stdin cases
• update the v1 spec and README to document the new i report stdin behavior
• no breaking changes

Additional notes

Trade-off: this change only fixes i report input assembly; it does not change the broader interactive confirmation or GitHub-auth behavior of the subcommand.

Intentionally deferred: broader i report UX policy cleanup and any stdin support changes for other subcommands.

Remaining gap: i report is now composable at the input boundary, but the rest of the subcommand still has separate follow-up opportunities around dry-run semantics and non-interactive confirmation behavior.

Closes #21

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