plugin marketplace add reports success but registration is not persisted (browse immediately fails with 'not found')

Describe the bug

copilot plugin marketplace add reports success, but the marketplace registration is never persisted anywhere on disk. A subsequent copilot plugin marketplace list (even in the same command chain / immediately after) does not show the newly added marketplace, and copilot plugin marketplace browse <name> fails with Marketplace "<name>" not found.

This reproduces both for a local directory marketplace and a GitHub-hosted marketplace, so it does not appear to be specific to local-path or git-auth handling (unlike #4103).

Environment

• GitHub Copilot CLI: 1.0.74 (latest at time of filing)
• OS: Windows 11 (PowerShell 7 / native)
• Marketplace manifest: valid .claude-plugin/marketplace.json at repo root (per docs, this location is supported)

Steps to reproduce

# Local directory marketplace with a valid .claude-plugin/marketplace.json
copilot plugin marketplace add C:\Git\Agentic
copilot plugin marketplace list
copilot plugin marketplace browse company-skills

# Also reproduces with a GitHub-hosted marketplace
copilot plugin marketplace add anthropics/claude-code
copilot plugin marketplace list

Actual result

Marketplace "company-skills" added successfully.

...but copilot plugin marketplace list afterward only shows the two built-in marketplaces:

Included with GitHub Copilot:
  - copilot-plugins (GitHub: github/copilot-plugins)
  - awesome-copilot (GitHub: github/awesome-copilot)

And:

Fetching plugins from marketplace "company-skills"...
Failed to browse marketplace: Error: Marketplace "company-skills" not found

repro-marketplace.zip

Confirmed via Compare-Object of a full recursive file listing under ~/.copilot, %LOCALAPPDATA%, and %APPDATA% before/after running add — no files are created or modified anywhere as a result of the command, despite the CLI reporting success.

Expected result

copilot plugin marketplace add should persist the marketplace registration so that a subsequent list/browse in a new process reflects it, matching the documented behavior in https://docs.github.com/en/copilot/how-tos/copilot-cli/customize-copilot/plugins-finding-installing.

Additional context

Possibly related to #3403 (~/.copilot/config.json being overwritten/reset on session start, dropping previously-written keys like hooks) — if marketplace registrations are also expected to live in config.json, the same overwrite-on-start bug could explain why the registration never survives past the single add invocation.

--- This report was investigated and drafted by GitHub Copilot CLI (Claude Sonnet 5).

Additional notes: name of marketplace changed for to company-skills. Additional details

Start at the CLI entry points for plugin marketplace add, list, and browse, then inspect the persistence location, including the possible ~/.copilot/config.json path and .claude-plugin/marketplace.json manifest. Reproduce with both the local directory and GitHub-hosted examples; done means registration survives a new process and appears in list and browse.