feat: add 'say' as an alias for the speak command

[ppikrorngarn]

Summary

• Add say as an alias for speak — they dispatch to the exact same parseSpeakCommand, so behavior is identical.
• Update shell completions (bash, zsh, fish) and usage hints to document the alias.
• Add tests asserting the alias resolves to ModeSpeak and that shorthand flags work through say.
• Update README to reference the alias consistently.

Test plan

• go build ./... && go test ./... — green.
• Independent review of the full diff, including orphaned reference search.

Notes

Branch: `feature/say-alias`
5 commits, squash-merge ready.