fix(packs): cover remaining gc session peek line-count syntax

[julianknutsen]

Maintainer follow-up for PR #1768

Original PR: #1768
Original title: fix(packs): use --lines flag for gc session peek (witness template)
Original state: MERGED
Configured base branch: main
Original GitHub base: main
Base mismatch: none

PR #1768 fixed the witness prompt's gc session peek usage after the CLI moved line counts to the --lines flag. The review found the same syntax pattern in nearby pack surfaces and requested a regression guard so this command-shape drift does not recur.

This follow-up contains only maintainer-side review fixes:

• update the remaining deacon prompt example to gc session peek <target> --lines 50
• update the shutdown-dance formula's gc session peek calls to use --lines
• add a focused packlint regression test for positional gc session peek <target> <n> syntax under examples/

This preserves the original contribution from #1768 and carries the maintainer-side review fixes through a separate merge surface because the original PR had already merged before finalization.

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[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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