fix(providers): clear timeout error message and fallback on CLI provider timeout

When a CLI provider subprocess was killed due to request_timeout, the error
reported was the raw subprocess signal ("signal: killed"), giving the user
no indication that a timeout was the cause. Additionally, the fallback chain
would not trigger because ClassifyError used == to match context.DeadlineExceeded,
which does not match wrapped errors.

- Each CLI provider now checks ctx.Err() == context.DeadlineExceeded after
  cmd.Run() fails and returns a descriptive error (e.g. "claude cli timed out
  after 30s") that wraps context.DeadlineExceeded
- ClassifyError updated to use errors.Is instead of == when checking for
  context.DeadlineExceeded, so wrapped timeout errors correctly classify as
  FailoverTimeout and trigger fallback to the next candidate in the chain

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pkg/providers/claude_cli_provider.go

@@ -63,6 +63,9 @@ func (p *ClaudeCliProvider) Chat(
 	cmd.Stderr = &stderr
 
 	if err := cmd.Run(); err != nil {
+		if ctx.Err() == context.DeadlineExceeded {
+			return nil, fmt.Errorf("claude cli timed out after %s: %w", p.timeout, context.DeadlineExceeded)
+		}
 		stderrStr := strings.TrimSpace(stderr.String())
 		stdoutStr := strings.TrimSpace(stdout.String())
 		switch {

pkg/providers/codex_cli_provider.go

@@ -86,6 +86,9 @@ func (p *CodexCliProvider) Chat(
 	}
 
 	if err != nil {
+		if ctx.Err() == context.DeadlineExceeded {
+			return nil, fmt.Errorf("codex cli timed out after %s: %w", p.timeout, context.DeadlineExceeded)
+		}
 		if ctx.Err() == context.Canceled {
 			return nil, ctx.Err()
 		}

pkg/providers/error_classifier.go

@@ -2,6 +2,7 @@ package providers
 
 import (
 	"context"
+	"errors"
 	"regexp"
 	"strings"
 )
@@ -114,7 +115,7 @@ func ClassifyError(err error, provider, model string) *FailoverError {
 	}
 
 	// Context deadline exceeded: treat as timeout, always fallback.
-	if err == context.DeadlineExceeded {
+	if errors.Is(err, context.DeadlineExceeded) {
 		return &FailoverError{
 			Reason:   FailoverTimeout,
 			Provider: provider,

pkg/providers/gemini_cli_provider.go

@@ -63,6 +63,9 @@ func (p *GeminiCliProvider) Chat(
 	cmd.Stderr = &stderr
 
 	if err := cmd.Run(); err != nil {
+		if ctx.Err() == context.DeadlineExceeded {
+			return nil, fmt.Errorf("gemini cli timed out after %s: %w", p.timeout, context.DeadlineExceeded)
+		}
 		stderrStr := strings.TrimSpace(stderr.String())
 		stdoutStr := strings.TrimSpace(stdout.String())
 		switch {