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/README.md

@@ -56,6 +56,7 @@ Language versions: [English](README.md) · [Português (pt-BR)](pt-BR/README.md)
 - [Commands](COMMANDS.md) — every command with flags and examples
 - [Configuration](CONFIGURATION.md) — full config schema, model profiles, git branching strategies
 - [CLI tools](CLI-TOOLS.md) — `gsd-tools.cjs` programmatic API for workflows and agents
+- [JSON error mode](json-errors.md) — `gsd-tools` failure channels: faults (stderr, exit 1) vs degraded results (stdout, exit 0), and the reason-code taxonomy
 - [Features](FEATURES.md) — complete feature index
 - [Inventory](INVENTORY.md) — installed skills and surface map
 - [STATE.md schema](reference/state-md.md) — field-by-field reference for `.planning/STATE.md`