fix(lint): handle resp.Body.Close() errcheck in keychain.go

[DreadPirateRobertz]

Summary

• Wraps deferred resp.Body.Close() to explicitly discard error, fixing errcheck lint violation
• This was the last remaining fix from PR fix: resolve all 14 golangci-lint errors on main #2210 not yet on main — all other fixes from that PR have been independently applied

Test plan

• go build ./cmd/gt passes
• go vet ./internal/quota/... passes

Supersedes the remaining content of #2210 (which can be closed after this merges).

🤖 Generated with Claude Code

[codecov-commenter]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
7862	1	7861	35

View the top 1 failed test(s) by shortest run time

github.com/steveyegge/gastown/internal/rig::TestAddRig_UpstreamURL

Stack Traces | 0.08s run time

=== RUN   TestAddRig_UpstreamURL
    manager_test.go:1312: AddRig: Dolt server is not running (required for beads init); start it with 'gt up' or 'gt dolt start'
--- FAIL: TestAddRig_UpstreamURL (0.08s)

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