fix(retention): clear the lint findings in the retention plugin

[juicycleff]

golangci-lint reports ten findings in plugins/retention on main, which
fails the ci / Lint job on every open PR regardless of what that PR touches.
It is currently red on #108, whose diff is three files and none of them here.

CI's output understates the problem

golangci-lint defaults to max-same-issues: 3, so it printed three bodyclose
findings where there were twenty-two. Fixing only the named lines surfaces
the next three. Worth knowing before trusting a pasted lint list as complete.

What changed

bodyclose, 22 sites. fakeResp builds a synthetic *http.Response for
classifyHTTPError to inspect. It has no Body: classification reads only the
status and headers, and nothing came from a real transport. The linter flags
the call sites because the function returns *http.Response, so they carry a
directive; closing a nil Body would panic. The reasoning sits on fakeResp
itself rather than being repeated 22 times.

errcheck, store_models.go. Job.Payload is a map[string]string, which
has no channel, func or cyclic value that could make json.Marshal fail.
Documented rather than given an error path that can never run.

govet shadow, provider_hubspot.go. The inner err in the PATCH branch
shadowed the one from findContactByEmail; renamed to patchErr.

revive unused-parameter, 6 handlers. Renamed r to _ where the handler
ignores the request — only those. The other handlers in these files do read
r, and renaming them all breaks compilation.

unparam, consent_test.go. Every caller passed purpose as "marketing",
so the parameter carried no information. Dropped, with the value bound as a
local const.

unused, provider_generic.go. Dropped classifierPolicyDecided, which no
longer gated anything, and kept its rationale as a comment. The point it makes
is worth keeping: an unwritten retry policy that only surfaced at the first CRM
error would decide by accident whether a transient 503 permanently drops a
customer's sync.

Verification

Full-repo golangci-lint run ./... on CI's exact version (2.12.2): 0 issues.
All 98 packages build and test clean.

Merging this should turn ci / Lint green on #108 and anything else currently
open.