zcode: fix user-scope install path to ~/.zcode/agents (per ZCode docs)

The user-scope dest was ~/.config/zcode/agents/, but the official ZCode docs
state subagents are read from ~/.zcode/agents/<name>.md — so user-scope
installs landed where ZCode never looks. (detect.agentsDir and project scope
were already correct at .zcode/agents, so this was an internal inconsistency.)

Point tools.json dest.user, install_zcode's default, the header comment, and
the list display at ~/.zcode/agents; drop the stale ~/.config/zcode detection
clause; update the integration README. Keep format `zcode-md` distinct — ZCode's
native format supports color/model/permissions, so it will diverge from gemini-md
as the converter matures rather than being a permanent alias.

Verified: a default user-scope install now writes to ~/.zcode/agents/<slug>.md
(not ~/.config/zcode); check-tools.sh passes at 16 tools; install.sh syntax and
tools.json JSON both valid.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01WKnDRWM4izsB8WAXKszhsq

integrations/zcode/README.md

@@ -24,6 +24,6 @@ Run the installer from your target directory:
 cd /your/project && /path/to/agency-agents/scripts/install.sh --tool zcode
 ```
 
-Agents install to `~/.config/zcode/agents/<slug>.md` (user scope). Use
-`--division` / `--agent` to install a subset, or set `ZCODE_AGENTS_DIR` to
-override the destination.
+Agents install to `~/.zcode/agents/<slug>.md` (user scope) — the directory
+ZCode reads subagents from. Use `--division` / `--agent` to install a subset,
+or set `ZCODE_AGENTS_DIR` to override the destination.