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>

docs/reference/host-integration-interface.md

@@ -27,7 +27,7 @@ value (or the `undocumented` sentinel):
 |---|---|
 | `embeddingMode` | `imperative` \| `declarative` |
 | `commandSurface` | `slash-file` \| `slash-programmatic` \| `slash-toml` \| `palette` \| `prose-only` |
-| `dispatch` | struct: `{ namedDispatch, nested, maxDepth, background, subagentToolkit, backgroundDispatch }` |
+| `dispatch` | struct: `{ namedDispatch, nested, maxDepth, background, subagentToolkit, backgroundDispatch, isolation, maxConcurrency }` |
 | `modelMode` | `active` \| `passive` |
 | `hookBus` | `host` \| `engine` \| `none` |
 | `stateIO` | `filesystem` \| `sandboxed-storage` \| `session-log-append` |
@@ -35,6 +35,19 @@ value (or the `undocumented` sentinel):
 | `runtime` | `node` \| `bun` \| `sandboxed-web` \| `python` \| `go` \| `rust` \| `electron` \| `other` |
 | `effortSurface` | `argv` \| `none` |
 
+`dispatch.isolation` (`harness-worktree` \| `orchestrator-worktree` \| `none`, added #2584)
+and `dispatch.maxConcurrency` (a positive integer, added #3673) are two dispatch
+sub-fields with their own per-field fail-closed floors — unlike the other five
+dispatch fields, they are not gated on `namedDispatch`. `dispatch.maxConcurrency`
+degrades to `1` (strictly sequential) on anything other than a positive safe
+integer (missing, the `undocumented` sentinel, zero, negative, fractional, an
+unsafe integer, or a non-numeric type); there is no negotiated host/engine
+reduction the way `maxDepth` has one — the resolved value passes through
+unchanged. Its live transport (`GSD_DISPATCH_MAX_CONCURRENCY`) is read only at
+the CLI query layer (`gsd-tools query dispatch-capacity`), never inside this
+pure negotiation module, and takes precedence over the descriptor value, which
+in turn takes precedence over the `1` fallback.
+
 ## Classification + negotiation
 
 - `profileOf(axes)` → `'programmatic-cli'` \| `'declarative-cli'` \| `'ide'` \| `null`.