hot-reload: argument errors bypass the structured error envelope; consider a changed-files default for --files

[hatayama]

Background (usability feedback, R21)

A tester ran uloop hot-reload without --files and got:

{
  "Methods": [], "Warnings": [], "PatchedTotal": 0, "ActivePatchTotal": 0, "UnchangedTotal": 0,
  "ClearedCount": 0, "AddedFields": [],
  "Message": "Files is required unless --revert-all or --status is set. Pass project-relative .cs paths with --files, e.g. 'uloop hot-reload --files Assets/Scripts/Player.cs'.",
  "Success": false
}

The message itself is good (includes an example). Two asks:

1. Error envelope consistency: other commands (find-game-objects, simulate-keyboard) return a structured error with Error.ErrorCode / Error.NextActions, while hot-reload argument errors reuse the success-shaped payload with Success: false. Callers cannot rely on one uniform error-detection rule (Success vs presence of Error).
2. --files default: when omitted, defaulting to "all .cs files changed since the last compile" would remove the need to retype paths the caller obviously knows.

Why deferred

Unifying the error envelope touches the hot-reload response shape broadly; the --files default is a behavior change worth its own design pass. Reported severity was low (3/10) and recovery was one round-trip thanks to the message quality.

[hatayama]

Folding #2337 into this issue: it tracks the same changed-files default for --files (auto-target .cs files changed since the last compile) and records the design concerns — implicit selection can patch files the agent did not intend, and mtime heuristics are fragile across git operations; the design pass must settle explicit opt-in vs default and how the candidate set is bounded. This issue now owns both asks: (1) structured error envelope for argument errors, (2) the changed-files default.