fix: [ENG-2880] address PR #687 review — JSON envelope, this.error, --, doc

Four items from the code review:

- Helper API redesign: `assertNoRemovedFlags` (which threw a bare
  `Error`) is now `findRemovedFlagMessage` returning `string | undefined`.
  Caller drives the response — JSON envelope when `--format json` is
  set, `this.error(...)` otherwise. Yields the canonical oclif `›
  Error:` rendering instead of a raw stack-leaking throw.
- JSON-mode envelope: both `brv curate --format json` and
  `brv query --format json` now emit a structured rejection envelope
  on a removed flag instead of dumping a plain-stderr line. Detected
  with a new `argvRequestsJsonFormat(argv)` helper that mirrors the
  recognised forms (`--format json`, `--format=json`).
- `--` terminator: the scan now stops at `--`, matching POSIX
  convention. Same applies to the JSON-format detector. Lets users
  on permissive-parse `Query` pass a literal query that contains a
  hyphen-prefixed token without hitting a false-positive rejection
  (e.g. `brv query -- what does --timeout do`).
- Stale SKILL.md `-f` / `--files` references: line 639 (the "File
  access" paragraph that described the removed --files flow) and
  lines 658–660 (error-table rows for `Maximum 5 files allowed` /
  `File type not supported`) replaced with a single row pointing
  agents at the new migration-error message shape. The other three
  doc files were already clean.

Tests
- 15 unit cases for the helper (was 8; +7 for `--` terminator,
  --flag=value, and argvRequestsJsonFormat).
- Full suite: 8521 passing, 16 pending, 0 failing.
- Typecheck clean. Lint 0 errors / 262 pre-existing warnings.