chore: clear golangci-lint findings in production code

Summary

Fixes all 14 golangci-lint errcheck findings and 1 staticcheck finding in production (non-test) Go files. Test-file findings (31 remaining) are handled in a follow-up MR that also flips allow_failure: false.

Before: 45 total findings (41 errcheck, 4 staticcheck) After: 31 findings — 0 in production files, 31 in test files (expected, next MR)

Findings fixed

File	Line	Linter	Resolution
cmd/manifold/run.go	95, 101	errcheck	defer os.RemoveAll → defer func() { _ = os.RemoveAll(...) }() — best-effort temp-dir cleanup
cmd/manifold/seed.go	41	errcheck	fmt.Scanln → _, _ = fmt.Scanln(...) — best-effort interactive prompt read
internal/analyzer/analyze.go	27, 28, 32, 33, 37	errcheck	tmp.Close / os.Remove in error paths → _ = — best-effort cleanup before returning primary error
internal/analyzer/analyze.go	359	errcheck	defer f.Close() on read-only file in readJSON → defer func() { _ = f.Close() }()
internal/client/client.go	488	errcheck	io.Copy(io.Discard, ...) drain → _, _ = io.Copy(...) — best-effort connection reuse
internal/collector/collector.go	160–174	errcheck	tmp.Close / os.Remove in error paths → _ = — best-effort cleanup
internal/renderer/render.go	177, 183	errcheck	defer in.Close() / defer out.Close() → defer func() { _ = ... }() — read-only src; explicit flush close on line 188 is the real one
internal/server/server.go	47, 51	errcheck	fmt.Fprint(w, ...) in HTTP handler → _, _ = fmt.Fprint(...) — client disconnect in handler is not actionable
internal/config/config.go	184	staticcheck QF1003	if ch == ' ' / else if ch == '\t' → tagged switch ch with labeled break indentLoop to preserve loop-exit semantics

Validation

• go vet ./...: pass
• go test -race ./... (CGO_ENABLED=1, Go 1.24.2): all 9 packages pass, no race conditions
• golangci-lint run (podman, v2.11.4): 0 production findings, 31 test-file findings (scope of next MR)
• allow_failure: true on the CI lint job is unchanged — will be flipped in the follow-up MR

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