feat: automatic Company completion in AI Code vterm sessions

[tninja]

Summary

• Add an opt-in, agent-independent Company completion mode for AI Code vterm sessions, including Claude Code and Codex.
• Reuse recorded prompt candidates and optional Cape dictionary words. Track ordinary typing and backspace; accept with Tab through the terminal adapter without submitting.
• Drop tracked input on unknown edits, avoid /commands and @file tokens, and verify terminal text before replacing it. Fix the Cape merged CAPF invocation.

Verification

• ERT: 7 terminal completion tests, 51 prompt completion tests, 163 terminal infrastructure tests passed.
• Byte compilation of changed Lisp files passed with warnings treated as errors; checkdoc checked the new module.
• Live Claude Code and Codex TUI sessions have not been exercised in this environment. Please verify timing, popup placement, Tab acceptance, CLI native menus, and paste behavior in each before merging.

Closes #515 after live validation.