feat(#3673): add dispatch.maxConcurrency axis and dispatch-capacity query (#4162)

* test(#3673): add failing tests for dispatch.maxConcurrency axis and dispatch-capacity CLI route

Extends tests/host-integration.test.cjs with negotiateHostCapabilities
maxConcurrency negotiation coverage (test matrix rows 1-15, including a
fast-check property test) and a new #3673 dispatch-capacity CLI route
describe block spawning the real gsd-tools.cjs (rows 16-25). Extends
tests/host-integration-validator-parity.test.cjs with an all-19-descriptor
maxConcurrency presence/validity sweep (row 26) and adds a hostile-input
validator test to host-integration.test.cjs (row 27).

The dispatch.maxConcurrency field does not exist yet, so these tests fail.

* feat(#3673): add dispatch.maxConcurrency axis, negotiation, validator parity, and the dispatch-capacity query

Adds a numeric dispatch.maxConcurrency sub-field to the Host-Integration
Interface (ADR-1239 Phase 1), following the existing dispatch.isolation
sub-field pattern: DispatchCapability interface, SAFE_DEFAULTS/PROFILE_BASELINES
floors, and a negotiateHostCapabilities branch that passes through a positive
safe integer and fails closed to 1 otherwise (no engine-side reduction, per
the design doc's explicit rejection of a min(host,engine) rule).

capability-validator.cjs gains parity validation for the new field (optional,
positive safe integer or the "undocumented" sentinel — mirroring isolation's
"added after existing descriptors" treatment).

gsd-tools.cjs gains a new `query dispatch-capacity` route, a pure-read sibling
of `dispatch-isolation` with no side effects: live env
(GSD_DISPATCH_MAX_CONCURRENCY) > descriptor > fallback-to-1 precedence.

All 19 capabilities/*/capability.json descriptors now declare
dispatch.maxConcurrency: claude carries the one cited value (20, per
code.claude.com/docs/en/sub-agents); the other 18 carry "undocumented"
(not yet researched for this axis).

* docs(#3673): document dispatch.maxConcurrency and add its citation row to the capability matrix

Updates docs/reference/host-integration-interface.md's dispatch struct entry
(also backfilling the previously-undocumented isolation/backgroundDispatch
sub-fields found stale in the same table) and adds fail-closed/live-transport
precedence prose for the new maxConcurrency field.

Adds a dispatch.maxConcurrency row (with citation) to all 19 host sections in
docs/reference/host-integration-capability-matrix.md — required for
tests/host-integration-descriptors.test.cjs's kimi-code matrix-parity check,
which asserts every declared dispatch sub-axis is documented there.

Updates docs/how-to/add-or-update-a-host-integration.md's dispatch checklist
and example descriptor block to mention maxConcurrency (and, likewise
backfilling a stale gap, isolation/backgroundDispatch).

* fix(#3673): extract shared maxConcurrency validator, drop dead reserved-name check

Exports isPositiveSafeInteger from src/host-integration.cts as the single
source of truth for the dispatch.maxConcurrency positive-safe-integer
contract; negotiateHostCapabilities and gsd-tools.cjs's routeDispatchCapacity
now both call it instead of independently reimplementing the same predicate.

Also removes the __proto__/constructor/prototype reserved-name branch from
capability-validator.cjs's maxConcurrency check — copy-pasted from the
string-enum fields above it, but unreachable for a numeric field (the
generic positive-safe-integer branch already rejects any string) and absent
from maxDepth, the field the code's own comment claims to mirror.

---------

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

capabilities/cursor/capability.json

@@ -80,7 +80,8 @@
         "background": true,
         "subagentToolkit": "full",
         "backgroundDispatch": true,
-        "isolation": "harness-worktree"
+        "isolation": "harness-worktree",
+        "maxConcurrency": "undocumented"
       },
       "modelMode": "passive",
       "hookBus": "host",