docs(#2980): ratify the payload-carried error idiom as a degraded result (#3270)

* docs(#2980): ratify the payload-carried error idiom as a degraded result

Records ADR-2980: an `error` key in a gsd-tools result payload on stdout with
exit 0 is a ratified contract meaning the command ran to completion and is
reporting a condition, not a process failure. Faults keep stderr + exit 1 +
--json-errors.

Normalizing the 42 output({error}) sites to exit 1 was declined on measured
blast radius (get_impact rates cmdStateSnapshot CRITICAL; output has 170 direct
callers) — a Hyrum's Law break with no versioning escape hatch for an exit code.

Adds the "Degraded results vs faults" section to docs/json-errors.md with a
correct-caller recipe, indexes that page from docs/README.md, and records the
two-channel contract in the CONTEXT.md I/O Module entry. No code change.

Closes #2980

* docs(#2980): correct the site count and cross-refs after review

The isolated review found the population figure was the answer to a regex,
not to the question. `output\(\{\s*error:` only matches literals whose FIRST
key is `error`; re-deriving it by brace-matching output()'s first argument
gives 60 sites across 9 modules (42 error-first + 18 error-not-first), adding
workstream.cts, phase.cts and gsd2-import.cts. roadmap.cts:260 is the case in
point — it carries `error` alongside `found:false` and is the site that
actually produces the documented `roadmap get-phase` output.

Also from review: correct the --raw claim (11 sites pass a rawValue, not 2),
reconcile the missing-required-argument count to the 7 verified sites, link
the bare ADR-2966 references per the ADR lifecycle rule, and fix 'licence'
to American spelling.

---------

Co-authored-by: sim <sim@local>

docs/adr/README.md

@@ -183,6 +183,7 @@ These govern the system as it stands. Cite these.
 | [ADR-2782](2782-reviewer-lane-capability-surface.md) | Reviewer Lane — the cross-AI reviewer handoff becomes a declared capability surface | Accepted | — |
 | [ADR-2866](2866-install-surface-resolution.md) | Install-surface resolution — the install pipeline resolves `(runtime × scope × trigger)` as a value | Accepted | — |
 | [ADR-2966](2966-loop-qa-walk.md) | Test the five-step loop as a continuous walk, not isolated points | Accepted | — |
+| [ADR-2980](2980-payload-carried-error-is-a-degraded-result.md) | A payload-carried `error` key is a degraded result, not a fault | Accepted | — |
 | [ADR-3180](3180-planning-semantic-model-single-owner.md) | Planning Semantic Model — Single Owner per Derivation | Accepted | — |
 | [ADR-3212](3212-lexical-seam-consolidation.md) | The Lexical Seam — Safe Pattern Construction, Line-Terminator Normalization, and Tokenizer-First Stateful Grammars | Accepted | — |
 | [ADR-3660](3660-runtime-artifact-layout-module.md) | Runtime Artifact Layout Module owns per-runtime artifact placement | Accepted | [ADR-1239](1239-gsd-embeddable-orchestration-engine.md) |