discovery: preload sentinel no longer distinguishes preload-fired from fallback-read (follow-up to #2338)

[kyle-sexton]

What happens

#2338 established that skills: preload does not reach dispatched discovery:researcher agents, and PR #2374 fixed the consequence: the agent now recovers by reading SKILL.md (+ context files) from disk, so the discipline runs either way.

That fallback creates a new problem: the recovered agent echoes the same preload_token a preloaded agent would, because the token is in the file it just read. A matching sentinel in the return payload no longer proves preload fired — it proves only that the agent saw the file by some route. Any parent or gate treating the token as preload evidence is unsound.

Evidence

Nine independent discovery:researcher dispatches on 2026-08-17 (CLI v2.1.232, this repo): 9/9 preload failures, 9/9 successful fallbacks, 9/9 matching tokens. Every agent disclosed the provenance — but only as free prose in open_questions, e.g.:

"The sentinel is echoed above because I read it from the skill file, not because it was preloaded. If the parent treats a matching token as proof the preload worked, that inference is wrong for this run."

Suggested direction

1. Make provenance structured, not prose: a preload: fired | fallback field in the return payload contract, graded by the parent, with the bare token demoted to file-identity evidence only.
2. Separately, the underlying preload failure from discovery:researcher skill preload does not reach dispatched agents #2338 is still live at v2.1.232 and now has a 9-run reproduction; worth deciding whether to reopen upstream investigation (agent-frontmatter skills: on the dispatch path) or record it as accepted-with-fallback.

Related

• discovery:researcher skill preload does not reach dispatched agents #2338 (the preload failure; closed via the fallback in fix(discovery): researcher must not assume skills preload succeeded #2374)
• docs/topics/context-budget/FINDINGS.md — "Corrections owed to this repository" item 5 records the 9/9 observation

[kyle-sexton]

Claimed by lane cursor/2895-preload-sentinel-cfcf: structured preload: fired | fallback provenance; demote matching token to file-identity evidence. Follow-up to #2338.