Replace the literal <cmd> placeholder in usage-error hints

[Widthdom]

Summary

Suggestions usage errors expose the internal literal placeholder <cmd> instead of an executable help command. The defect appears in both human and structured JSON errors.

Environment

• origin/main: c642bc4c3dda4ef659df8e6b699e10f61da3c836
• cdidx v1.44.3, locally built
• full net8.0 and net9.0 suites pass

Reproduction

Human output:

dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll suggestions list \
  --bogus --db .cdidx/codeindex.db

Error: --bogus is not supported for suggestions.
Hint: Run '<cmd> --help' for usage information.

Structured output:

dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll suggestions show \
  --db .cdidx/codeindex.db --json

{
  "message": "suggestions show requires an id.",
  "hint": "Run '<cmd> --help' for usage information."
}

The same placeholder also appears in option-compatibility errors such as using --actor outside suggestions update.

Expected behavior

Use the nearest executable help surface, for example:

Run `cdidx suggestions --help` for usage information.

If command identity is unavailable, fall back truthfully to cdidx --help.

Root cause

• src/CodeIndex/Cli/CommandErrorWriter.cs:9 defines DefaultHint as the final literal Run '<cmd> --help' for usage information.
• Suggestions preflight and usage paths pass that constant directly.
• Some error-writer paths already know the canonical command, but the default-hint API does not consume it.

Focused PR scope

One PR should:

1. replace the literal constant with a command-aware hint builder;
2. pass canonical command identity from suggestions usage/preflight paths;
3. provide a safe global fallback;
4. keep human and JSON hints equivalent;
5. audit current callers for the same placeholder exposure;
6. add a bilingual changelog fragment.

Non-goals

• Do not redesign all CLI diagnostics.
• Do not infer commands from arbitrary error text.
• Do not expand this into help-content generation.

Implementation guidance and cautions

• Accept a canonical bounded command identifier, not untrusted free text.
• Use existing diagnostic quoting/formatting helpers.
• Do not perform a global post-serialization replacement.
• Preserve error codes, exit codes, categories, and usage blocks.
• Prefer the nearest actually supported help surface for nested commands.

Required tests

• Suggestions unknown option in human mode.
• Missing suggestions ID and invalid option combinations in JSON mode.
• Known-command and unknown-command fallback paths.
• Assert no user-facing output contains literal <cmd>.
• Preserve existing error/usage contracts.

Acceptance criteria

• No user-facing error emits <cmd>.
• Suggestions errors point to an executable help command.
• Human and JSON paths agree.
• Existing structured error contracts do not regress.

Duplicate preflight

No exact open or closed issue was found for the literal default-hint placeholder.