Every command crate's test module re-rolled the same ~30 LOC of
`SharedBytes` / `SharedWriter` / `Captured` / `make_output` glue —
about 350 LOC of pure boilerplate across 15 files, drifting over
time (some `Captured` had `stderr`, some didn't; one file even
carried a `_stderr_accessor_lives` dead-code stub just to silence
the resulting warning).

Extract the canonical version into a new `mergify-test-support`
crate that other crates pull in as a `dev-dependencies`. The new
`Captured` API exposes `human()` / `new(mode)` constructors and
`stdout()` / `stderr()` accessors, so the common pattern shrinks
from a 12-line `String::from_utf8(cap.stdout.lock().unwrap().clone())`
to `let s = cap.stdout()`.

Net `-418 / +114` lines across the workspace. Behavior unchanged;
all 233 tests pass.

The crate is `publish = false` and only ever appears under
`[dev-dependencies]`, so the test-only types never leak into a
production build.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Change-Id: If9d688cdaf55360ba90f386d2020b52346c19b28

Every `queue` and `freeze` command opens with the same four-line
ritual: resolve repository slug, resolve token, resolve API URL,
build a Mergify-flavored HTTP client. Wrap those into a
`CommandContext::resolve(...)` constructor + `ctx.mergify_client()`
builder living in a new `mergify_core::command_context` module.

The eight queue/freeze command preludes shrink from

    let repository = auth::resolve_repository(opts.repository)?;
    let token = auth::resolve_token(opts.token)?;
    let api_url = auth::resolve_api_url(opts.api_url)?;
    let client = HttpClient::new(api_url, token, ApiFlavor::Mergify)?;

to

    let ctx = CommandContext::resolve(opts.repository, opts.token, opts.api_url)?;
    let client = ctx.mergify_client()?;

`auth::resolve_*` stay public for the specialized callers that
don't fit the shape: `config validate` needs no repository,
`config simulate` derives the repo from a PR URL, `ci scopes-send`
resolves the repo from CI-env vars (different fallback chain).
Those keep wiring up the lower-level helpers by hand.

Net `-73 / +63` across 9 files; conceptually the bigger win is
that "the prelude of a Mergify command" is now a named concept
with one fix-it-once point.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
Change-Id: I1b272570f6802dd13438c3d9baf26709b196574d