feat(acp-snapshot): extract the ACP snapshot suite into a support package

The snapshot tier's machinery leaves examples/acp-agent/tests for
packages/support/acp-snapshot (@deepseek-ai/dsh-acp-snapshot), where the
coverage gate measures it and a second example can consume it instead of
forking it: harness.ts (runScenario, parameterized by an AgentUnderTest
{binScript, configPath, tsconfigPath} instead of module constants),
normalize.ts (moved verbatim), and suite.ts (defineAcpSnapshotSuite — the
per-scenario golden/log compares, record write-back, per-suite header pin
with its uniformity guard, and the fixture guard block, lifted from
acp.snapshot.ts). The example file collapses to its scenario table plus
one factory call; env reading (DSH_SNAPSHOT) stays at that edge.

The exactly-one-pin meta-test generalizes from the hardcoded text-turn
name to "exactly one per suite" — which scenario pins is the scenario
table's reviewable choice (per-suite pinning per the proposal RFC).

Extraction parity: pnpm run test:snapshot is 36 passed + fs-policy-reject
failing BEFORE AND AFTER (BSD-sed environment failure, reproduced at the
base commit in a clean worktree — the recorded golden's sed -i syntax is
GNU-only), with zero byte changes under examples/acp-agent/tests/snapshots/.
Coverage for the new src files lands in the next commit.

knip.json

@@ -9,7 +9,7 @@
         "examples/echo-agent/tests/**/*.e2e.ts",
         "examples/coding-agent/tests/**/*.e2e.ts",
         "examples/acp-agent/tests/**/*.e2e.ts",
-        "examples/acp-agent/tests/**/*.snapshot.ts"
+        "examples/*/tests/**/*.snapshot.ts"
       ],
       "project": ["scripts/**/*.ts", "examples/**/*.ts"]
     },
@@ -21,6 +21,11 @@
       "project": ["src/**/*.ts"],
       "ignoreDependencies": ["cordis"]
     },
+    "packages/support/acp-snapshot": {
+      "entry": ["tests/**/*.spec.ts"],
+      "project": ["src/**/*.ts", "tests/**/*.ts"],
+      "ignoreDependencies": ["cordis"]
+    },
     "packages/core/agent-loop": {
       "entry": ["tests/**/*.spec.ts", "tests/**/*.e2e.ts"],
       "project": ["src/**/*.ts", "tests/**/*.ts"]