fix(link): ibc keys * fixes

[swift1337]

• allow keys operations w/o config directory or config file
• --populate-config shorthand -p
• fail to populate config on existing key to reduce confusion and ambiguity

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[greptile-apps]

Greptile Summary

This PR allows key-management commands to operate without an existing config directory or file and centralizes flag-based configuration overrides.

• Adds optional config loading for the keys command family.
• Refactors signer insertion into Config.AddSigner.
• Changes existing-key and private-key handling for key creation/import.

Confidence Score: 3/5

The PR is not safe to merge until config load failures stop causing destructive fallback writes and existing keys can again be registered with --populate-config.

Optional loading can replace an existing unreadable or malformed operator config with defaults, while the new existing-key checks make the supported signer-registration repair flow unreachable.

Files Needing Attention: link/cmd/ibc/config.go, link/cmd/ibc/keys.go

Important Files Changed

Filename	Overview
link/cmd/ibc/config.go	Introduces optional loading but conflates an absent config with all load failures, allowing population to overwrite an existing invalid config.
link/cmd/ibc/keys.go	Moves validation skipping to the command hook and regresses the existing-key population workflow for both new and import.
link/internal/config/config.go	Extracts flag overrides and adds an immutable-style, duplicate-aware signer insertion helper.
link/cmd/ibc/main.go	Adds a shorthand and updates the description for the populate-config flag without an independent defect.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  A[Run keys new/import] --> B[Load optional config]
  B -->|Config absent| C[Default config]
  B -->|Config load error| C
  B -->|Loaded| D[Existing config]
  C --> E{populate-config?}
  D --> E
  E -->|Yes| F[Add signer]
  F --> G[Write complete config file]
  E -->|No| H[Operate on key only]

Loading

_{Reviews (1): Last reviewed commit: ee1ad8e | Re-trigger Greptile}

[linear-code]

FOU-1414

FOU-1127

[gjermundgaraba]

I think the optional config stuff can be done in a simpler way, see #1420 for a suggested improvement.