feat(desktop): migrate config from YAML to JSON format (spec 162)

[Copilot]

Standardize desktop config format from desktop.yaml to desktop.json, aligning with projects.json and eliminating format inconsistency across LeanSpec config files.

Changes

config.rs

• Changed CONFIG_FILE from "desktop.yaml" to "desktop.json"
• Switched serialization from serde_yaml to serde_json
• Added automatic migration: detects legacy YAML → converts to JSON → backs up as .bak
• Added logging for migration and backup operations

// Migration flow
const CONFIG_FILE: &str = "desktop.json";
const LEGACY_CONFIG_FILE: &str = "desktop.yaml";

// Try JSON first, fall back to YAML migration
if let Some(legacy_config) = load_legacy_yaml() {
    legacy_config.persist();  // Save as JSON
    backup_legacy_yaml();     // Rename to desktop.yaml.bak
    return legacy_config;
}

Documentation

• Updated packages/desktop/README.md with correct config paths
• Updated spec 148 config examples from YAML to JSON
• Updated spec 168 desktop config section from YAML to JSON

Notes

serde_yaml retained in Cargo.toml for migration support and spec frontmatter parsing in frontmatter.rs/projects.rs.

Original prompt

implement spec 162

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