[codex] support explicit command continuations

[brandonbloom]

Summary

• Add explicit greater-than command continuation lines for multiline shell commands.
• Record multiline commands back with continuation opcodes during update.
• Add matching heredoc, multiline quote, shell block, multiple heredoc, and update coverage.

Validation

• ./test.sh

[claude]

Claude encountered an error —— View job

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I'll analyze this and get back to you.

[brandonbloom]

Decision note: this explicit continuation design wins out over #5.

The extra continuation marker is a little more verbose, but it keeps .cmdt grammar deterministic and reviewable. Readers can tell which lines are command input without mentally running shell parsing, and fixture lines that look like transcript opcodes remain unambiguous.

[claude]

Claude encountered an error —— View job

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I'll analyze this and get back to you.