Bug: /model command does not persist agent model change to .jsonc config (1.18.10)

Description

The /model slash command updates the in-memory agent model immediately (behavior changes live), but the change is not persisted to disk when the config file uses the .jsonc extension. After restart, the old model is loaded from the unchanged config file.

Steps to Reproduce

1. Start opencode with config at ~/.config/opencode/opencode.jsonc
2. Run /model, select an agent (e.g., plan), pick a different model
3. Observe: agent behavior changes immediately (in-memory update works)
4. Check config file mtime: stat --format='%n | mtime: %y' ~/.config/opencode/opencode.jsonc
5. Observe: mtime does not change — the file on disk was not written
6. Restart opencode
7. Observe: agent reverts to the old model from disk

Root Cause

In internal/config/config.go, the UpdateAgentModel function (line ~874) calls updateCfgFile (line ~836) to persist the change. updateCfgFile resolves the config file path via:

configFile := viper.ConfigFileUsed()
if configFile == "" {
    configFile = filepath.Join(homeDir, ".opencode.json")
}

Problem 1: Viper is configured to look for .opencode.json (line ~218: viper.SetConfigType("json")), but the user's file is .opencode.jsonc. If opencode loads .jsonc via custom logic, viper.ConfigFileUsed() returns "", so the write falls back to ~/.opencode.json — a different file than the real config.

Problem 2: updateCfgFile parses the file with json.Unmarshal, which is not JSONC-aware. If the .jsonc file contains // comments, the parse fails and the write is silently skipped.

Expected Behavior

/model should persist the model change to the same config file that was loaded, regardless of extension (.json or .jsonc).

Workaround

Edit opencode.jsonc directly: set agent.<name>.model to the desired model, then restart opencode.

Version

opencode 1.18.10

Suggested Fix

1. updateCfgFile should resolve the config file path identically to the initial config loader (which handles .jsonc)
2. Use a JSONC-aware parser instead of json.Unmarshal for reading/writing
3. The fallback path should match the actual loaded config location, not a hardcoded ~/.opencode.json