Do not emit option metavariables as shell-completion enum values

[Widthdom]

Summary

PowerShell completion suggests literal name/path values for free-form options such as --project and --open-issues, because placeholders are treated as finite enum choices.

Audit IDs: P-10. Reproduced with locally built cdidx v1.40.3 at 404db09489c305f17d94f37026e00564d3ac527a.

Existing issue relationship

Residual/follow-up adjacent to closed #1807, #4347, #4732, #4571, and #4427's completion/option metadata work.

Expected behavior and PR scope

Canonical option metadata must distinguish metavariables from finite choices; every shell generator should consume that distinction. One completion-regeneration PR.

Implementation guide for Codex 5.6 sol xhigh

• Represent value kind (enum/path/project/repo/free text/etc.) separately from placeholder display text.
• Regenerate all supported shell completions and add source-level/smoke assertions across shells.
• Provide contextual path/project/repo completion where supported without offering placeholder literals.
• Cover aliases, multi-value options, optional values, quoting, and dynamic value sources.
• Add focused regression/contract tests, run the full net8.0 and net9.0 suite, and add the required bilingual changelog.d/unreleased/ fragment.

Acceptance criteria

• name/path are never suggested as literal enum values for free-form options.
• Real finite values remain available.
• All supported shell scripts use the same canonical metadata.

[Widthdom]

Work started on branch fix-issue4902.