Generate suggestion category completions from the accepted registry

[Widthdom]

Summary

Runtime validation accepts newer suggestion categories, but generated Bash, zsh, fish, and PowerShell completions still advertise only the older category set.

Accepted but omitted values include:

• security
• performance
• bug
• cleanup
• documentation
• feature_request

Reproduced with locally built cdidx v1.45.1 at 81861a14c9396cd74190edc91392ab11abfa86f8.

Reproduction

dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll suggestions add \
  "dogfood" --category security --db <db> --json

dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll help suggestions add
dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll completions bash

The add command and detailed help recognize 14 categories. Generated completions expose the older eight.

Expected behavior

Parser validation, help/schema introspection, MCP metadata, and all generated shells expose the same authoritative category set and stable order.

Existing issue relationship

This is a regression/follow-up to:

• Expand suggestion categories for security, performance, bug, cleanup, and feature findings #4423, which added the expanded suggestion categories;
• Make suggestions add --help command-specific and enumerate constrained values #4422, which documented/discovered category values;
• Add contextual enum-value completions consistently across all shells #4426, which required enum completions across all shells.

Root-cause pointers

• src/CodeIndex/Models/SuggestionRecord.cs:175-190 contains the canonical accepted set.
• src/CodeIndex/Cli/CliFlagSchema.cs:644 contains a stale hard-coded subset.
• src/CodeIndex/Mcp/McpToolCatalog.cs:831-832 already consumes the canonical set.

PR scope and implementation guidance

• Define the public category domain once and share it across validation, help, CLI schema, MCP metadata, and shell renderers.
• Avoid a per-shell patch or another copied list.
• Preserve stable serialized spellings, especially feature_request.
• Add a contract test proving every advertised category is accepted and every accepted public category is advertised.
• Keep stored suggestions and export/import compatibility unchanged.

Acceptance criteria

• Runtime, root/command help, schema metadata, MCP, and all four generated shells expose the same category set and order.
• Every suggested category succeeds in suggestions add.
• Invalid categories remain rejected with bounded guidance.
• Existing persisted records and exports remain compatible.

Tests, documentation, and changelog

• Add schema-registry parity tests and syntax/behavior checks for Bash, zsh, fish, and PowerShell output.
• Run the complete net8.0 and net9.0 test suites.
• Update suggestions/completion documentation if category lists are shown.
• Add the required bilingual changelog.d/unreleased/ fragment referencing Expand suggestion categories for security, performance, bug, cleanup, and feature findings #4423 and Add contextual enum-value completions consistently across all shells #4426.

[Widthdom]

Work started on branch fix-issue5235.