feat(method): refuse prompt boilerplate as unique content

[seonghobae]

Instruction and prompt boilerplate stays explicit method structure (ADR 0004/0012). It is not unique latent content and is not erased by a stopword list. Recovery is the computed share of recovered kinds that match known truth versus collapsing every token to unique content.

Complementary to #75 report-template inferential-weight, #147 section, #148 style, #149 copied-text, #150 modality, and #151 corpus-background. This crate owns prompt-versus-unique-content identity.

Local gates:

• `cargo test -p prompt_source --all-targets` GREEN after RED (package did not exist)
• clippy `-D warnings` PASS
• workspace contract PASS
• docstring contract PASS
• lines 14/14; nightly-2026-08-01 branches 6/6

Does not allocate migration `0008`. Does not recreate those in-flight method-source crates.

Keep this PR draft. Preferred merge remains #47 only when exact-head required Checks pass and a qualifying independent APPROVE exists. Do not empty-commit #47. Re-draft #144 if it is undrafted.

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