ADR 0148: Strand detection covers every gate stage

Amendments.

• Vocabulary: current pointers use gate capability / custom check → known/custom job; the decision and reasoning are unchanged.
• ADR 0262 — earlier stop: receipt-eligible runs now stop at the pre-group strand checkpoint, moving this check earlier for that class.

Status: accepted; extends ADR 0047, works with ADR 0094 and ADR 0116

ADR 0047 made done block when the fix stage strands changes on a committed-clean tracked file, and deliberately scoped the check to that one stage. The fix stage is the stage meant to mutate; CI's trailing git diff --exit-code stood as the guard for tracked mutations from non-fix stages.

Field use showed that residual gap has the exact failure mode ADR 0047 set out to remove. A project whose build stage (or a test, or a scope gate) regenerates a tracked artifact — a project generator rewriting a tracked manifest, a codegen step rewriting tracked schemas — dirties the tree outside the fix-stage snapshot window. The gate goes green, the receipt silently refuses (skipped_dirty), and the only signal is a hint riding on a green result — the "advisory on green" pattern ADR 0047 itself rejected as the thing agents ignore. An agent watching a long gate paid the full run, got an ambiguous green, blamed the strand on "the formatter", and paid a second full run after a diagnosis loop. CI's diff guard never enters that local loop.

discern's own repository carries the same shape: its build job runs the codegen task, which rewrites tracked schema, type, and reference files.

done blocks when any stage group strands changes on a previously clean tracked file, and the diagnostic names the stage that did it.

• The snapshot window widens from one stage to all. The gate captures the tracked-dirty set once before any stage group runs and again after each green group (fix, build, check/test, scope gates). The stranded set keeps its spirit: paths dirty in the final snapshot but not at gate start — so a stage reworking the agent's own uncommitted edits (the inner loop) still never trips, and a path a later stage restores to its committed state does not count (the finished tree is what the receipt vouches for).
• Each strand is attributed to the first snapshot that shows it. The diagnostic says which stage produced each file — "the build stage", "a scope gate" — replacing the formatter-shaped guesswork the old green path invited. The failed-stage label is renamed fix_drift → tree_drift to match the widened class; the diagnostic tool label is tree-drift.
• Everything else about ADR 0047 stands. Tracked changes only (an untracked file a stage emits is visible in git status and remains a legitimate pattern); a final gate check that runs only when the gate is otherwise green; block-don't-autocommit; no toggle; the capped git diff and commit-then-re-run rescue in the diagnostic; fail-open when a snapshot cannot be read.

• "Green but no receipt" from a gate-caused tracked mutation no longer exists. The failure moves from a quiet skipped_dirty hint after a full run to a red gate with the diff and the origin stage in hand — same worst-case two runs, but the first run explains itself.
• The wire vocabulary changes: failed_stage: "tree_drift" replaces "fix_drift". Schemas and types regenerate from the one vocabulary source; consumers switching on the closed set get a compile error until they handle the new label, which is the enrolment working as designed.
• A gate job that legitimately regenerates tracked files now requires the regenerated output committed before done passes — true for this repository's own codegen build step. That is the discipline ADR 0094 already asks for at accept time, surfaced at the point the agent is looking.
• Cost: one extra git status per green stage group (three to four per gate run), negligible beside the jobs themselves.
• The remaining honest gap is unchanged from ADR 0047: a stage emitting a new untracked file still leaves the worktree dirty for the receipt without blocking the gate — visible in git status, disclosed by the receipt hint (which now names the dirty paths), and outside CI's diff guard too.

• Keep the fix-stage scope and improve the green-run hint. Rejected for the same reason ADR 0047 rejected advisory hints: agents key on ok, and a hint on a green gate is what gets ignored — that was the observed failure.
• Snapshot only at gate start and end (no attribution). Cheaper by two git status calls, but the diagnostic could not name the origin stage, and a wrong guess at the origin ("the formatter did this") is precisely the diagnosis loop this change removes.
• A separate failed-stage label per origin stage. Rejected: the class is one invariant (a green gate must leave the tracked tree as it found it); the origin belongs in the diagnostic, not the vocabulary.
• Auto-commit the stranded output. Rejected again per ADR 0047/0094: a gate is not a committer, and discern cannot know the commit boundary or message.