style: adopt golangci-lint v2 and resolve all findings

Add .golangci.yml (v2): the standard linters plus bodyclose, errorlint, noctx, misspell, rowserrcheck, sqlclosecheck, unconvert, usestdlibvars, with gofumpt + goimports formatters. Enable the std-error-handling exclusion preset for idiomatic Close/Remove/Setenv ignores; scope-exclude SA1019 (parser.ParseDir in tools/openapigen) and ST1005 (intentional capitalized user-facing error copy that tests assert verbatim). No inline nolint directives were introduced.

Resolve all 217 findings behavior-preserving: gofumpt/goimports formatting, explicit blank assignment on intentionally ignored errors, errors.Is/errors.As and %w wrapping, context-aware stdlib calls (CommandContext/QueryContext/NewRequestWithContext/Dialer), staticcheck simplifications, removed redundant conversions, http.StatusOK and http.MethodGet, inlined the go:fix intPtr helper, and deferred sql rows Close. Add a golangci CI job mirroring the existing Go jobs.

internal/mtproto/process.go

@@ -5,6 +5,7 @@
 package mtproto
 
 import (
+	"context"
 	"errors"
 	"fmt"
 	"os"
@@ -154,7 +155,7 @@ func (p *Process) Start() error {
 	if p.IsRunning() {
 		return errors.New("mtg is already running")
 	}
-	cmd := exec.Command(GetBinaryPath(), "run", p.configPath)
+	cmd := exec.CommandContext(context.Background(), GetBinaryPath(), "run", p.configPath)
 	cmd.Stdout = p.logWriter
 	cmd.Stderr = p.logWriter
 	p.cmd = cmd