fix: keep senior-dev help within eighty columns

[ZeroPoint95]

Keep senior-dev help within eighty columns and check carried help on every PR.

What changed

• Shortened --verify-build, --verify-test and --state-dir descriptions.
• Referred state-directory defaults to the existing manual.
• Added the existing carried-help width test to the CI light gate and local test-quick.
• Documented the gate coverage.

How it was checked

1. Reproduced the width failure on the exact dev base: 137/101/100 columns.
2. Passed the width test and go test ./internal/delegate/... ./internal/seniordev -count=1.
3. Passed the deadline/midstream cancellation regression with -count=100.
4. Built with make build; independent review drove real help in an isolated 80-column tmux: maximum width 78.
5. Passed BASE=524843e0aa279f8a677f08572623a3817e95bd97 make pr-ready with readonly modules and a temporary official flock utility. The Mac's missing flock also failed the original base; namespace arms report their usual missing-bwrap skip.

Checklist

• Change entry: 1810-senior-dev-help-width.md.
• Existing manual remains accurate; CI documentation updated.
• No known-red entry added.
• Only this repair's paths committed.