* feat(#3908): the scanners distinguish an empty diff from one they could not compute
collect_files ended 2>/dev/null || true, which destroyed the evidence three ways: the redirect discarded git's diagnostic, the pipe replaced git's status with grep's, and || true forced success regardless. Four distinct conditions - an established-empty diff, a bad ref, no repository, and a repository with no commits - all reported clean, and a secret scanner reporting clean because git failed is indistinguishable from an all-clear to any gate consuming it.
git now runs separately from the filter so its status and diagnostic both survive. An established-empty diff exits NO_INPUT; a scope that could not be established exits UNAVAILABLE; the usage sites move off 2 to USAGE. || true is retained on the filter alone, where it is correct: a diff of only images is empty, not failed.
Codes are sourced from a generated shell fragment rather than written into three scripts, so a re-allocation cannot desync them, and a missing fragment fails loudly instead of falling back to literals. The security workflow is updated in the same change: without it, a docs-only PR would newly fail the job.
* fix(#3908): keep scanner stderr out of the file list, and drop try/finally from test bodies
Capturing git and find output with 2>&1 was right for the failure path but wrong for the success path: a warning emitted alongside a successful diff flowed into the file list and was treated as a filename. stderr is now captured separately, forwarded as a warning on success and as the diagnostic on failure, and never folded into the list.
Also converts the control tests' try/finally blocks to t.after(), which CONTRIBUTING bans inside a test body because it masks failures.
* chore(#3908): backfill changeset pr number
* docs(#3908): record the scanners' four-outcome exit contract
SECURITY.md is root-level, so the docs gate correctly held: a Changed fragment owes a file under docs/. The contract also belongs where the feature is described, as REQ-SCAN-INJ-05.
docs/FEATURES.md is GENERATED from per-feature fragments (#3840) - the first edit went into the generated file and gen-features --check caught it, which is the same edit-the-output drift this epic exists to close. The fragment is the source; FEATURES.md is regenerated.
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Co-authored-by: sim <sim@local>
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