feat: zero fmt --write and --check --json envelope

[gke0op]

Summary

Adds --write flag and --json envelope support to zero fmt, matching the contract of every other Zero command.

New features

• zero fmt --write <file> overwrites the file in place
• zero fmt --check --json emits structured envelope with ok and unformatted
• zero fmt --write --json emits structured envelope with ok and written
• --check and --write are mutually exclusive (error in both text and json modes)
• Bare zero fmt still prints to stdout (backward compatible)

JSON envelope shape

{
  "schemaVersion": 1,
  "command": "fmt",
  "mode": "check",
  "sourceFile": "examples/hello.0",
  "ok": true
}

When unformatted:

{
  "schemaVersion": 1,
  "command": "fmt",
  "mode": "check",
  "sourceFile": "/tmp/messy.0",
  "ok": false,
  "unformatted": ["/tmp/messy.0"]
}

Changes

• Added fmt_write field to Command struct
• Added --write flag parsing
• Added mutual exclusivity check for --check and --write
• Added JSON envelope for --check mode (was previously ignored)
• Added JSON envelope for --write mode
• Updated help text (both summary and detailed)
• Text mode behavior unchanged

Testing

All existing tests pass (9 CLI, 8 docs, command contracts).

Manual verification:

• zero fmt --check examples/hello.0 → exit 0
• zero fmt --check /tmp/messy.0 → exit 1 with "format differs"
• zero fmt --check --json /tmp/messy.0 → envelope with ok:false, exit 1
• zero fmt --write /tmp/messy.0 → rewrites file, prints "formatted: "
• zero fmt --write --json /tmp/messy.0 → envelope with ok:true
• zero fmt --check --write file.0 → mutual exclusivity error
• zero fmt file.0 → still prints to stdout (backward compat)
• Formatted output is idempotent (fmt → check → ok)

Closes #9

[vercel]

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