Added:
- Version constraint support (`version-constraints` in config) with SemVer matching (exact, ranges, ^, ~, wildcards)
- `/depo tree` command to display per-plugin dependency declarations (shows which plugins declare each dependency, missing status, applied constraint)
- Interactive conflict resolution via `/depo resolve <dep> <ignore|relax|override>`
- Plugin categories grouping in `/depo status` (config `categories:`)
- Download progress output (config `download-progress` with throttled console feedback for larger downloads)
- Soft dependency management: separate optional dependency set, `/depo soft` command (list/install), and `auto-download-soft` config (default false)
- Numbered soft dependency list with multi-select install by indices or names; log hint suggesting `/depo soft list` when optional dependencies are missing

Changed:
- Config version bumped to 4 through staged migrations:
- v2: add `version-constraints` map
- v3: add `categories`, `download-progress`
- v4: add `auto-download-soft`
(Automatic migration preserves existing values and appends new keys.)

Internal:
- Exposed dependency scanning maps & install function for command handlers
- Improved message key auto-merge to preserve user customizations

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