feat(#2092): migrate Qwen Code onto EoS imperative adapter + native subagents + SubagentStart (ADR-1239)

Fold all runtime==='qwen'/isQwen logic branches (skill-priority frontmatter,
branding/path rewrites, legacy commands/gsd cleanup, hyphen-namespace
normalization, RUNTIME_CONTENT_DISPATCH, hooks-surface label) into
descriptor-driven runtime.hostBehaviors on capabilities/qwen/capability.json,
read via _hostBehaviors(). Shared claude/qwen/hermes legacy-migration branches
in install-engine.cts folded to descriptor flags (claude+hermes descriptors
updated; FALLBACK_HOST_BEHAVIORS.claude floored). Byte-identical golden parity
for qwen/hermes/claude(global+local).

UPGRADE 1: native .qwen/agents/*.md subagent projection — new agents
artifact-layout kind + convertClaudeAgentToQwenAgent converter (name +
description + tools YAML block list; color/model dropped). qwen routed onto
the descriptor-driven agents path (_DESCRIPTOR_AGENTS_RUNTIMES).
UPGRADE 2: SubagentStart hook wired into extendedHookEvents + the
descriptor-gated hook-writer loop (activates only for qwen).

Tests: qwen-imperative-reference (adapter/axes/fail-closed/hostBehaviors +
no runtime==='qwen' source-grep across 4 files) + qwen-upgrades (agents file
validity + SubagentStart mirrors SubagentStop, descriptor-gated). Docs matrix
+ how-to updated; changeset added.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/reference/host-integration-capability-matrix.md

@@ -365,6 +365,8 @@ Documentation gaps:
 - dispatch.nested — docs only restrict fork-type sub-agents from nesting; whether named sub-agents can themselves spawn named sub-agents is not stated.
 - dispatch.maxDepth — depth=1 is documented only for fork sub-agents; depth for named sub-agent chains is undocumented.
 
+**EoS migration status (#2092):** Migrated onto the imperative adapter. All `runtime === 'qwen'` branches in `bin/install.js`, `src/install-engine.cts`, `src/runtime-artifact-conversion.cts`, and `src/runtime-hooks-surface.cts` folded into descriptor-driven `runtime.hostBehaviors`. Two upgrades land: (1) **native subagent projection** — a new `agents` artifact-layout kind projects GSD's specialist agents into `~/.qwen/agents/gsd-*.md` as native Qwen subagents via `convertClaudeAgentToQwenAgent`, emitting Qwen's own `name:`/`description:`/`tools:` (YAML block list) frontmatter schema instead of Claude Code's; cite https://qwenlm.github.io/qwen-code-docs/en/users/features/sub-agents/. (2) **`SubagentStart` hook** — wired into `extendedHookEvents` alongside the existing `SubagentStop`/`Stop`/`PreCompact` events, firing the context-monitor hook symmetrically at subagent start and completion; cite https://qwenlm.github.io/qwen-code-docs/en/users/features/hooks.
+
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 ## codebuddy