fix(cli): auto-detect piped stdin without --stdin flag (#13)

[puemos]

Closes #13

Problem

git diff develop | lareview --agent claude opened an empty GUI and did nothing. Stdin was only read when the explicit --stdin flag was passed.

Solution

Read stdin once up front via try_read_stdin_diff() (already non-destructive when stdin is a TTY). When no other source matches and the pipe has content, build a PendingDiff with source: "stdin". Explicit flags (--stdin, refs, --pr, --status) still take precedence.

Refactored process_cli_args out of src/main.rs into src/infra/cli/args.rs as a pure function taking piped_stdin: Option<String>, so the logic is unit-testable without real IO.

Test plan

• cargo test --lib infra::cli::args — 4 tests, incl. issue-13 repro
• cargo test — 205 passed, 8 suites
• cargo clippy -- -D warnings — clean
• Manual: git diff main | lareview --agent claude opens GUI with diff pre-loaded
• Manual: lareview --stdin --agent claude < f.diff still works
• Manual: lareview with no args still opens empty GUI
• Manual: lareview main feature still opens with git-diff request