docs(plans): retire the promise-linked work plan

[gcko]

What

Deletes docs/plans/promise-linked-work.md and docs/plans/promise-linked-work-implementation.md. Both were transient plan docs, and AGENTS.md says a plan is deleted once its work ships. All eight work packages have landed:

• Tasks 1 to 6 merged as docs(promise-map): promise IDs and the plan for promise-linked work #250 and docs(promise-link): the machinery reads the promise link (work packages 2 to 6) #253: the promise map defines the vocabulary, and the burndown skill, both Spacedock workflows, the board, sync-docs and the release skill read it.
• Task 7 and 8 were Linear-only: five move:* labels exist, the project description carries the two-labels paragraph and a by-move count, and all 47 open issues carry a journey:* and a move:* label with a ## User value section.

The durable content is already where it belongs. The vocabulary and the rule that only extend and new change the map's wording live in docs/promise-map.md. The one alternative considered and rejected, re-ranking the board on promise moves above the blind panel's score, is recorded here for the reader who wonders: it was set aside because the panel scored the build order and a re-rank would need its own audit, and the board README now states the division of labour instead.

Verification

python3 scripts/validate_plugins.py passes, and nothing in the repository references either file.

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