Flatten opencode config to ~/.ucode/opencode.json and reclaim the orphaned opencode-xdg folder

[larsmoan]

Background

#387 stopped ucode redirecting XDG_CONFIG_HOME and named its config file with
OPENCODE_CONFIG instead. That fix leaves two things behind.

1. The directory is still called opencode-xdg, but ucode no longer exports it
   as an XDG config home. The name describes a job that no longer exists.
2. OpenCode installed @opencode-ai/plugin into that folder while the redirect
   was active — about 62 MB in ~/.ucode/opencode-xdg/opencode/node_modules.
   After fix(opencode): honor the user's own opencode config #387, OpenCode uses ~/.config/opencode/ again, so that copy is unused.

OPENCODE_CONFIG names the file directly, so the two nested directory levels
no longer serve any lookup rule. The honest layout is a flat file beside the
backup that already lives there:

~/.ucode/
├── state.json
├── opencode.json                  ← moved here
├── opencode-config.backup.json    ← already at this level
└── opencode-xdg/                  ← removed, reclaims ~62 MB

Do not force a reinstall

The MCP server list lives in ~/.ucode/state.json under mcp_servers; ucode
renders it into opencode.json. The render is diff-gated in
apply_mcp_server_changes — it writes only when the server list changes. A path
change does not change state, so re-running ucode mcp add writes nothing. A
user could not repair a lost render by re-running the command. So the port must
carry the file across, not re-render it.

Plan

Move the file once, on the launch path, then delete the old folder.

_LEGACY_CONFIG_PATH = APP_DIR / "opencode-xdg" / "opencode" / "opencode.json"

def _adopt_legacy_config() -> None:
    if _LEGACY_CONFIG_PATH.exists() and not OPENCODE_CONFIG_PATH.exists():
        OPENCODE_CONFIG_PATH.parent.mkdir(parents=True, exist_ok=True)
        _LEGACY_CONFIG_PATH.replace(OPENCODE_CONFIG_PATH)

Call it at the top of write_tool_config, write_mcp_server_config, and
remove_mcp_server_config. Then remove ~/.ucode/opencode-xdg/ once the file
is adopted, to reclaim the 62 MB. A move beats a re-render: it works offline, it
carries every key rather than only the ones state knows, and it stays
deterministic. The not OPENCODE_CONFIG_PATH.exists() guard makes it idempotent
and stops a new file being overwritten.

Tests

• legacy file present, new file absent → file adopted, folder removed
• both present → new file kept, legacy folder removed
• neither present → no-op

Open item to confirm first

Verify where OpenCode installs node_modules once the config file sits at
~/.ucode/opencode.json. Evidence from #387 says OpenCode still treats
~/.config/opencode/ as its config home (it reads permissions and
disabled_providers from there), so the install should land there, not back in
~/.ucode/. Confirm with one run before deleting the old folder in code.

[larsmoan]

Not worth tracking. The leftover node_modules is a one-time, harmless artifact; no action needed.