refactor(#1277): T1 — decouple agent-install-check + git-base-branch from the core spine (#1280)

First leaf-migration tranche of epic #1267 (after T0 #1268). Migrate the
via-core callers of the two leaves T0 created to import from the leaf
modules directly, and stop core re-exporting their symbols:
- checkAgentsInstalled: docs.cts, verify.cts, init.cts -> agent-install-check.cjs
- gitWorktreeInfoInternal: init.cts -> git-base-branch.cjs
- getAgentsDir had no external via-core caller (internal to the leaf)

core no longer re-exports getAgentsDir / checkAgentsInstalled /
gitWorktreeInfoInternal; the now-unused agent-install-check + git-base-branch
requires are dropped from core; the shim-identity assertions for these are
deleted (behaviour tests retained). Convergence lint stays green (0 new).
No behaviour change.

Closes #1277

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

src/verify.cts

@@ -28,6 +28,9 @@ import { PACKAGE_NAME } from './package-identity.cjs';
 import { formatGsdSlash, resolveRuntime } from './runtime-slash.cjs';
 import { detectSchemaFiles, checkSchemaDrift } from './schema-detect.cjs';
 import { isCanonicalPlanningFile } from './artifacts.cjs';
+// eslint-disable-next-line @typescript-eslint/no-require-imports -- agent-install-check.cjs is an export= CommonJS module
+import agentInstallCheck = require('./agent-install-check.cjs');
+const { checkAgentsInstalled } = agentInstallCheck;
 
 const {
   loadConfig,
@@ -40,7 +43,6 @@ const {
   extractCurrentMilestone,
   output,
   error,
-  checkAgentsInstalled,
   CONFIG_DEFAULTS,
   inspectWorktreeHealth,
 } = core;