mutagen sync list: add structured output format (e.g. --output json)

[ksemele]

Third-party tools and IDE integrations currently have to parse the human-readable text output of mutagen sync list. This is fragile and expensive to maintain. A structured output flag (analogous to kubectl get ... -o json or docker inspect) would make it straightforward to build reliable integrations.

Motivation

I'm building a VS Code extension source code that polls mutagen sync list on a configurable interval to display session state in a tree view and status bar. The current parser has to:

• Split the output on ---...--- separator lines
• Track indentation level to distinguish root fields from Alpha: / Beta: / Conflicts: sub-sections
• Pattern-match the Status: string against a hardcoded list of substrings ("staging", "reconciling", "scanning", "applying", "saving", "waiting") to classify the session as syncing
• Parse Connected: Yes/No as a boolean

Any change to the output format — new fields, reworded status strings, indentation changes — silently breaks the parser.
The categorizeStatus() function in particular is a maintenance liability: it has to be updated every time a new status phase is introduced.

Proposed solution

Add a --output (or --format) flag to mutagen sync list:

mutagen sync list --output json
mutagen sync list --output json <session-name>

The JSON output should include the same information currently shown in the text output:

[
  {
    "name": "my-session",
    "identifier": "...",
    "status": "watching",           // machine-readable enum
    "statusDescription": "Watching for changes",  // human-readable, as today
    "paused": false,
    "alpha": {
      "url": "...",
      "connected": true
    },
    "beta": {
      "url": "user@host:/path",
      "connected": true
    },
    "lastError": null,
    "conflicts": []
  }
]

Key asks:

• status as a stable machine-readable enum (not a display string) — this is the most important part. Right now statusDescription changes phrasing between versions; a separate enum field would be stable across releases.
• connected as a proper boolean, not "Yes"/"No" string
• Empty array for conflicts when none (not omitting the field)
• Exit code 0 with [] when no sessions exist (currently outputs "No sessions found." to stdout)

Prior art

kubectl get pods -o json
docker inspect <container>
gh pr list --json number,title,state
terraform show -json

All of these kept their human-readable default output and added --output json as an opt-in. Same approach would work here with no breaking changes.