chore: adopt Go library-quality standard (format, lint, coverage, docs)

[AndroidPoet]

Summary

Brings the project up to a Go library-quality baseline across four pillars: formatting, linting/static analysis, test-coverage tooling, and a documentation website. No runtime behavior changes — the source edits are lint/format fixes that preserve existing logic.

Pillars

1. Formatting — gofumpt

• Adopted gofumpt as the canonical formatter and reformatted the whole codebase.
• make fmt (write) and make fmt-check (CI-style gate) targets added.
• CI fails if anything is not gofumpt-formatted.

2. Linting / static analysis — golangci-lint

• Expanded .golangci.yml (schema v2) from 2 linters to a sensible default set: govet, staticcheck, errcheck, ineffassign, unused, plus bodyclose, noctx, nilerr, errorlint, gocritic, prealloc, whitespace, misspell, unconvert, revive, with gofumpt + goimports as formatters.
• Fixed every surfaced issue (≈65): unchecked errors, exec.Command → exec.CommandContext, capitalized error strings, a deprecated strings.Title call, a dead assignment, prealloc, and more.
• CI runs golangci-lint via the official action.

3. Test coverage

• go test ./... -coverprofile=coverage.out -covermode=atomic wired into a make cover task that emits both an HTML report and a per-function summary (plus make cover-func).
• CI adds a coverage step and uploads coverage.out as an artifact.
• Note: existing test coverage is low (only internal/cli and internal/output have tests). This PR wires up the tooling; broadening test coverage is intentionally out of scope and left as follow-up.

4. Docs site

• New Next.js docs site under website/ (static export, served from GitHub Pages under the repo sub-path).
• Project-specific content written from scratch: Introduction, Installation, Getting Started, Usage, Command Reference (Setup & Auth, Catalog, Customers & Subscriptions, Analytics & Reports, Operations), Configuration, and Output & Scripting.
• The docs CI workflow now builds and deploys the static export; the previous docs pipeline and its sources were removed to avoid two competing systems.

Verification (run locally)

Check	Result
gofumpt -l .	empty (clean)
go build ./...	pass
go vet ./...	pass
go test ./...	pass
golangci-lint run ./...	0 issues
make cover	HTML + func report produced
docs pnpm build	16 static pages exported

Notes

• No pillar skipped. Coverage tooling is fully wired; raising the coverage number is deliberately follow-up work.
• go mod tidy promoted golang.org/x/text to a direct dependency (used to replace deprecated strings.Title).