Add gosec + correctness linters to golangci-lint

[KARTIKrocks]

Enable a stronger linter set befitting a security-adjacent tool: gosec (SAST), errorlint, bodyclose, nilerr, unconvert, usestdlibvars.

Annotate the deliberate patterns gosec/nilerr flag, each with a scoped //nolint and a justification rather than a blanket disable:

• explain: EXPLAIN string concatenation (G202) — no bind params exist; the defense is validate() + the rolled-back read-only tx.
• config.Load: reading the .sqlguard.yml path the caller pointed at (G304).
• pg/mysql parsers: return the best-effort fallback Statement (not an error) on parse failure, by contract (nilerr).
• scanner: skip an unparseable Go file and keep walking (nilerr).

Exclude gosec from _test.go (lax fixture perms / ignored setup errors), mirroring the existing errcheck exclusion. gosec G115 is excluded globally (noisy on deliberate range-checked int conversions).

Summary

What does this PR change, and why?

Closes #

Type of change

• Bug fix
• New detection rule
• New integration / parser
• Feature / enhancement
• Docs only
• Refactor / chore

Checklist

• make ci passes (fmt-check, vet, lint, test-race) across all modules
• Added/updated tests (and, where practical, a failure-mode check)
• Updated docs as needed (README.md, CLAUDE.md, .sqlguard.example.yml)
• Added an entry under ## [Unreleased] in CHANGELOG.md
• No new third-party deps in analyzer / middleware / reporter
• Findings stay redaction-safe (no raw literals leak into a Result)

Notes for reviewers

Anything reviewers should focus on — tricky areas, trade-offs, follow-ups.

Summary by CodeRabbit

• Chores
  • Expanded linting and security checks to catch more correctness, error-handling, and security issues during development.
  • Refined lint rules and exclusions so tests and examples are handled more appropriately.
  • Added clearer guidance around cases where invalid SQL or unparsable files are skipped instead of stopping scans.

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Review details

⚙️ Run configuration

Configuration used: Path: .coderabbit.yaml

Review profile: ASSERTIVE

Plan: Pro Plus

Run ID: 2a522d76-8cb2-40db-a186-87b94458fe1b

📥 Commits

Reviewing files that changed from the base of the PR and between b29c907 and 4c3da20.

📒 Files selected for processing (6)

• .golangci.yml
• cmd/sqlguard/scan.go
• config/config.go
• explain/explain.go
• parsers/mysqlparser/mysqlparser.go
• parsers/pgparser/pgparser.go

Walkthrough

This PR expands .golangci.yml with additional security and correctness linters (gosec, errorlint, bodyclose, nilerr, unconvert, usestdlibvars), tunes linter settings, and broadens test-path exclusions. It adds //nolint directives with explanatory comments to existing intentional patterns in scan, config, explain, and parser code, without changing logic.

Changes

Lint hardening and suppressions

Layer / File(s)	Summary
Linter configuration updates .golangci.yml	Enables gosec and related linters, sets gocyclo/revive/gosec options, excludes G115, and extends test-path exclusions to cover both errcheck and gosec.
Parser fallback nolint annotations parsers/mysqlparser/mysqlparser.go, parsers/pgparser/pgparser.go	Adds //nolint:nilerr and expanded comments documenting non-fatal parse-failure fallback behavior; no logic change.
Scan skip-on-parse-error annotation cmd/sqlguard/scan.go	Adds //nolint:nilerr and comment explaining unparseable files are skipped during AST scan.
EXPLAIN and config path gosec suppressions explain/explain.go, config/config.go	Adds //nolint:gosec suppressions (G202, G304) with comments explaining safety guarantees from validate() and read-only transactions, and the caller-provided config path intent.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Poem

A rabbit hops through lint rules tight,
Sprinkling nolint comments left and right 🐰
Gosec, nilerr, all in a row,
Explaining why the concat's okay to go.
No logic bent, just clarity's light.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title accurately summarizes the main change: enabling gosec and related correctness linters in golangci-lint.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch chore/security-linters

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[KARTIKrocks]

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