fix(#1400): flush agent-skills block via writeAllSync instead of process.exit(0)

cmdAgentSkills' plain (non-JSON) path wrote the <agent_skills> block with
process.stdout.write then immediately called process.exit(0). stdout.write is
async on pipes/files, so on Windows the process tore down before Node flushed
the buffer — the workflows' `$(gsd_run query agent-skills <type>)` capture
received 0 bytes and every ${AGENT_SKILLS_*} substitution expanded empty,
silently dropping configured per-agent skills.

Route the plain path through the existing synchronous output() helper
(writeAllSync, src/io.cts) + return — the same flush-safe mechanism the --json
branch already uses — instead of write + process.exit(0).

Adds a #1400 regression block to tests/agent-skills.test.cjs that captures
stdout via a real file descriptor and asserts the block is non-empty and
byte-identical to the --json .block.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/init.cts

@@ -2104,10 +2104,14 @@ function cmdAgentSkills(
     return;
   }
 
-  if (block) {
-    process.stdout.write(block);
-  }
-  process.exit(0);
+  // #1400: emit the raw block via the synchronous-flush output() helper (the same
+  // one the --json branch uses) rather than process.stdout.write + process.exit(0).
+  // When stdout is a pipe/file (how workflows consume this via command
+  // substitution) the async stdout buffer is torn down by process.exit() before
+  // it drains — on Windows this reliably truncates the write to 0 bytes, so every
+  // ${AGENT_SKILLS_*} substitution expands empty. output() writes every byte with
+  // writeAllSync and returns, letting the event loop drain naturally.
+  output(block || '', true, block || '');
 }
 
 interface SkillEntry {