[codex] support multiline command blocks

[brandonbloom]

Summary

• Accumulate commands until the shell parser reports they are complete.
• Support heredocs, multiline quoted strings, shell blocks, and multiple heredocs.
• Add checker/update coverage and docs.

Validation

• ./test.sh

[claude]

Claude encountered an error —— View job

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I'll analyze this and get back to you.

[brandonbloom]

I recommend closing this design in favor of #6.

The auto-detect approach is elegant and shell-like, but it makes the transcript parser depend on shell completeness. That means whether a line is a transcript opcode depends on prior shell syntax, error behavior depends on mvdan/sh parser details, and future transcript syntax changes become riskier because raw command bodies can silently absorb lines.

For a snapshot-test format, the explicit continuation opcode in #6 is easier to read and review. It keeps .cmdt visibly line-oriented: command input lines are marked as command input, while assertions stay assertions.

[brandonbloom]

Closing in favor of #6, which uses explicit command continuation lines.