chore(lint): enable 19 new linters and address violations

Summary

Enables 19 new golangci-lint linters that catch issues the existing config did not, and addresses all surfaced violations (auto-fix where possible, hand-fix the rest).

This is the companion to !3346 (merged) (the MR review-instructions expansion): many of the rules synthesized from @timofurrer's review feedback into that YAML are now enforced deterministically at edit time, freeing Duo Code Review to focus on judgment calls instead of repeating mechanical findings.

Linters added

Code-quality and bug-class:

• testifylint — testify-specific hygiene: require before indexing, assert.Empty over assert.Equal(t, "", x), no require in HTTP handlers, JSON unmarshal asserts.
• errorlint — wraps error comparisons with errors.Is/As; catches err == sentinel style.
• bodyclose — HTTP response bodies must be closed.
• contextcheck — context propagation through call chains.
• misspell — common typos in comments and string literals.
• forcetypeassert — flags unchecked type assertions (panics on mismatch).
• nilerr — flags if err != nil { return nil } (bug class).
• fatcontext — detects nested context creation in loops.
• asasalint — catches func(...any) accidentally invoked with a []any slice.
• gochecksumtype — exhaustive switch on sum-type interfaces.

Hygiene and safety:

• gocheckcompilerdirectives — guards //go: directive syntax.
• exptostd — flags x/exp packages now in the stdlib.
• gomoddirectives — go.mod directive hygiene.
• nolintlint — //nolint directives must be well-formed and used.
• reassign — flags reassignment of top-level package variables.
• nosprintfhostport — flags unsafe host:port URL construction (IPv6 bug class).
• asciicheck — bans non-ASCII identifiers (Unicode homoglyphs).
• bidichk — bans dangerous bidi unicode (CVE-2021-42574).

Plus the previously-disabled staticcheck check ST1005 is re-enabled, which enforces Go's standard error-string style (lowercase first letter, no trailing punctuation; see https://go.dev/wiki/CodeReviewComments#error-strings).

What this changes in the codebase

Type	Count
Files touched	~250
Auto-fixed by golangci-lint --fix	~165 files (mostly testifylint + errorlint)
Hand-fixed	~85 files
Test files updated to match new error strings	~25

A handful of files received small real improvements as a side effect of the linter probes:

• internal/commands/ci/ciutils/utils.go::getPipelineId now accepts a context.Context (was previously calling context.Background() mid-stack).
• internal/commands/ci/lint/lint.go::run now threads cmd.Context() through its HTTP request.
• internal/commands/api/api.go::run now closes paginated HTTP response bodies per iteration.
• 24 unchecked type assertions across 12 files now use the comma-ok form (or require.True in tests).
• One real nilerr bug fixed in agent_update_kubeconfig.go (the function previously swallowed os.Executable() errors and continued with an empty path). Four other nilerr hits are intentional fallback paths and carry //nolint:nilerr with a reason.

What was NOT included (deliberately)

• noctx: enforcing it surfaces a structural issue in internal/git, internal/browser, and parts of internal/iostreams that intentionally do not carry context.Context. Threading ctx through those packages is its own refactor; the bare exec / interactive launchers don't benefit from cancellation semantics. Tried, reverted; capture as a future follow-up if we want to enforce it.

Coordination with !3346 (merged)

!3346 (merged) (the review-instructions expansion) has been updated to drop the ~14 rules now enforced deterministically by linters here, so the two MRs are complementary rather than overlapping. Each remaining rule in !3346 (merged)'s YAML now has a one-line note pointing at which linter handles the basics, so Duo's review comments cite the right authority.

Other linters probed (not included, reported for follow-up)

Linter	Violations	Recommendation
gosec	73	Needs careful triage; security findings should not be batch-fixed
predeclared	45	Hard — most violations are package names (delete, list, etc.) that conflict with predeclared identifiers. Renaming packages is high-blast-radius.
gocritic	33	Opinionated; would need disabled-checks tuning. Mostly autofixable cosmetics.
goprintffuncname	2	IOStreams.StartSpinner / StopSpinner would need to be renamed to *Spinnerf. Breaking internal-API change.
durationcheck	1	False positive at internal/utils/utils.go:134 (m * time.Minute is the remainder-to-nanos idiom). Not worth a nolint.

Test plan

• CI lint job passes with the new .golangci.yml.
• CI test suite passes (locally: go test ./... is green).
• @timofurrer review on whether ST1005 error-string changes feel consistent with the codebase's voice (many were Capital. Sentence. before; now they're lowercase phrase).
• Run Duo Code Review on this MR itself to confirm it doesn't flag the same things the linters now catch.

Depends-on: !3346 (merged)