Refactor completions as native runtime functions

[DannyBen]

⚠️ This is a breaking change intended for the 2.0.0 release.

cc #657

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This PR replaces Bashly's Completely-based completion generation with native runtime completion functions embedded in the generated Bash script.

The generated CLI therefore remains the single source of truth for command, argument, and flag behavior. Shell completion wrappers can remain small and static while automatically reflecting changes made to the CLI.

Philosophy

1. Follow the runtime completion approach used by Go's Cobra framework.
2. Optionally generate a hidden yourcli __complete command that returns the valid candidates for an incomplete command line.
3. Keep shell-specific wrappers small and static. They only pass the shell's completion words to yourcli __complete and consume its response.
4. Generate or install these wrappers through bashly add completions, starting with Bash and potentially extending to Zsh and Fish.

Advantages

• The generated script is the single source of truth for both CLI behavior and completion candidates.
• Completion data no longer needs to be generated and maintained as a separate artifact.
• Installed shell wrappers automatically reflect CLI changes, without needing to be regenerated or reinstalled.
• Completion behavior can honor Bashly directives such as conflicts, repeatable values, and allowed values.
• Completely is no longer required as a runtime dependency.

Disadvantages

• Generated scripts are larger because they include the runtime completion functions.
• Each completion request invokes the generated CLI, adding some startup overhead compared with a fully static completion script.

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TODO

• Initial prototype implementation
• Complete native completion behavior
  • Commands
  • Arguments
  • Flags
• Establish runtime completion test architecture
• Remove the Completely integration and dependency
  • Remove bashly completions --install since it needs Completely
• Add a setting to enable completion generation
  • Update JSON schema
• Remove bashly add completions in favor of internal native function generation
• Revisit the completions directive in bashly.yml
• Examples
• Complete runtime completion coverage
• All tests pass
• Migration guide
• Documentation