chore: bump anthropic SDK to v1.21.0 and clean up unused dependencies

See https://www.anthropic.com/news/claude-opus-4-6

## CHANGES

- Upgrade anthropic-sdk-go from v1.20.0 to v1.21.0
- Add `ClaudeOpus4_6` to supported Anthropic model list
- Remove unused indirect dependencies from go.mod and go.sum
- Clean up legacy protobuf and gRPC version references
- Remove stale tensor, math, and image processing libraries
- Drop unused table writer and console dependencies
- Prune obsolete cloud.google.com version pins

internal/plugins/ai/anthropic/anthropic.go

@@ -40,6 +40,7 @@ func NewClient() (ret *Client) {
 		string(anthropic.ModelClaude3_7SonnetLatest), string(anthropic.ModelClaude3_7Sonnet20250219),
 		string(anthropic.ModelClaude3_5HaikuLatest), string(anthropic.ModelClaude3_5Haiku20241022),
 		// The following are the current supported models
+		string(anthropic.ModelClaudeOpus4_6),
 		string(anthropic.ModelClaudeOpus4_5_20251101),
 		string(anthropic.ModelClaudeOpus4_5),
 		string(anthropic.ModelClaudeHaiku4_5),