feat(upgrade): report each step's outcome as JSON for a program driving the upgrade

[imorland]

Fixes: no issue.

Changes proposed in this pull request:
upgrade 2.0 --json lets another program, such as an AI agent, drive the upgrade. It asks nothing (it implies -n) and prints one JSON object when it finishes, exiting non-zero if the upgrade didn't complete. For each step the object gives its status (committed, no-changes, skipped, failed or not-run). A committed step adds its commit, its manual-work notes and the @TODOs its commit added. A failed step lists the files it couldn't upgrade, each with the reason. Errors from before any step come back in the same shape.

Without --json, output that isn't going to a terminal no longer has cursor codes or spinner lines. The API resource stub's @todo is now @TODO, like every other marker, and a stray console.log in the frontend misc step is gone because it would break the JSON. The README now documents upgrade.

Reviewers should focus on:

• TODOs are read from each step's commit diff. A TODO line that the commit only reformatted or moved isn't counted, so the extension's own TODOs stay out of the list.

Screenshot
N/A

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered? (N/A)
• For core PRs, does this need to be in core, or could it be in an extension? (N/A)
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation. (The built CLI ran upgrade 2.0 --json on a real 1.x extension.)
• Frontend changes: tests are green (run yarn test in js/).
• Frontend changes: tests have been added, or are not appropriate here.
• Backend changes: tests are green (run composer test). (N/A)
• Backend changes: tests have been added, or are not appropriate here. (N/A)
• Where applicable, changes are suitable for all supported database drivers (MySQL, MariaDB, PostgreSQL, SQLite). (N/A)
• Core developer confirmed locally this works as intended.
• The description above is written by me and describes what this pull request actually does.