[STG-1738] feat: add upgrade command to browse-cli

[shrey150]

Summary

• Adds a browse upgrade command that updates the globally installed @browserbasehq/browse-cli to the latest version
• Auto-detects the user's package manager (npm/pnpm/bun/yarn) via npm_config_user_agent env var, falling back to checking which are available on PATH
• Runs the appropriate global install command with inherited stdio so the user sees progress

Test plan

• Run browse upgrade with npm installed globally
• Verify it detects the correct package manager
• Verify the upgrade command output is visible to the user

🤖 Generated with Claude Code

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Summary by cubic

Add browse upgrade to let users update the globally installed @browserbasehq/browse-cli to the latest version in one step. Runs the correct global install for the user’s package manager with visible progress.

• New Features
  • Detects package manager from npm_config_user_agent, falling back to checking what's on PATH.
  • Supports npm, pnpm, bun, and yarn.
  • Prints and executes the global install command with inherited stdio, then exits with the same status.

^{Written for commit fa7e822. Summary will update on new commits. Review in cubic}

[changeset-bot]

🦋 Changeset detected

Latest commit: fa7e822

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 0 packages

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[cubic-dev-ai]

No issues found across 2 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

Architecture diagram

sequenceDiagram
    participant U as "User"
    participant CLI as "browse-cli"
    participant Env as "Environment (Node.js)"
    participant OS as "OS / Shell"
    participant PM as "Package Manager (npm/pnpm/bun/yarn)"
    participant Registry as "Remote Registry (npm)"

    U->>CLI: browse upgrade

    CLI->>Env: NEW: Get npm_config_user_agent
    Env-->>CLI: User agent string (if exists)

    alt userAgent detected
        CLI->>CLI: Match pm (npm/pnpm/bun/yarn)
    else userAgent missing (fallback)
        loop through pnpm, bun, yarn, npm
            CLI->>OS: NEW: execFileSync("which", [candidate])
            OS-->>CLI: Path or error
            opt path found
                CLI->>CLI: Select candidate as pm
            end
        end
    end

    Note over CLI,PM: Command mapped (e.g., npm install -g @browserbasehq/browse-cli@latest)

    CLI->>PM: NEW: spawnSync(cmd, args, { stdio: "inherit" })
    
    activate PM
    PM->>Registry: Fetch latest version
    Registry-->>PM: Package data
    PM-->>U: Stream installation progress (stdout/stderr)
    deactivate PM

    PM-->>CLI: Return exit status
    CLI->>U: Exit with PM status code (0 or 1)

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