Improve runner prompt receipts

[rnbennett]

Outcome

• revises the unattended runner prompt with affirmative one-item, base-preservation, PR-disposition, shared-state, and terminal-stop constraints
• adds three worked terminal receipt examples covering reviewed PASS and two BLOCKED paths
• pins the prompt-quality gate and the intentional Claude shared-surface change

Verification receipts

Baseline focused runner contracts:

ℹ tests 6
ℹ pass 6
ℹ fail 0

Baseline prompt lint:

clarity: 0
examples: 0
total: 9
passing: false

TDD red:

ℹ tests 3
ℹ pass 2
ℹ fail 1
AssertionError: runner clarity must be >= 1, got 0

Final prompt lint:

clarity: 3
examples: 3
total: 15
passing: true

Final focused suite:

ℹ tests 35
ℹ pass 35
ℹ fail 0

Final broad suite (npm test):

ℹ tests 3447
ℹ suites 14
ℹ pass 3444
ℹ fail 0
ℹ cancelled 0
ℹ skipped 3
ℹ todo 0
ℹ duration_ms 330516.782829

npm run check:codex: ok: true.

Review gate

Reviewed exact commit: 39acd2684ed06a07a8870abdd1a4dc05f15f6773

VERDICT: PASS

Fix loops: 0.

Mutant-kill: changing the frontmatter action lead from Select back to Run made the new guard fail with runner clarity must be >= 1, got 0; committed bytes were then restored and verified byte-identical before review.

Source backlog item: improve-runner-prompt-examples.