Add OPENCODE_PORT environment variable for tmux integration

[alvinunreal]

Summary

Changes

[greptile-apps]

Greptile Summary

Added support for configurable OpenCode server port via OPENCODE_PORT environment variable with default fallback to 4096. This enables multiple OpenCode instances to run simultaneously on different ports with proper tmux integration.

Changes:

• Modified tmux session manager and utilities to read port from OPENCODE_PORT environment variable
• Updated tests to respect the environment variable for consistent behavior
• Enhanced documentation with clear setup instructions and examples for both default and custom port configurations

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• Simple, well-tested change that adds environment variable support with sensible defaults. All usages are consistent across codebase and tests, no breaking changes introduced
• No files require special attention

Important Files Changed

Filename	Overview
src/background/tmux-session-manager.ts	Added OPENCODE_PORT environment variable support for configurable default port (lines 45-46)
src/utils/tmux.ts	Added OPENCODE_PORT environment variable support for configurable port in error hint message (lines 202-205)
src/background/tmux-session-manager.test.ts	Updated test mock to respect OPENCODE_PORT environment variable for consistent test behavior (lines 20-27)
README.md	Updated documentation to reflect OPENCODE_PORT environment variable usage and improved setup instructions

Sequence Diagram

sequenceDiagram
    participant User
    participant Shell
    participant TmuxSessionManager
    participant TmuxUtils
    participant OpenCodeServer

    User->>Shell: export OPENCODE_PORT=5000
    User->>Shell: opencode --port 5000
    Shell->>OpenCodeServer: Start server on port 5000
    
    User->>TmuxSessionManager: Initialize plugin
    TmuxSessionManager->>TmuxSessionManager: Read OPENCODE_PORT env var (default: 4096)
    TmuxSessionManager->>TmuxSessionManager: Set serverUrl to http://localhost:5000
    
    TmuxSessionManager->>TmuxSessionManager: onSessionCreated(child session)
    TmuxSessionManager->>TmuxUtils: spawnTmuxPane(sessionId, config, serverUrl)
    
    TmuxUtils->>TmuxUtils: Check if server running at serverUrl
    TmuxUtils->>OpenCodeServer: GET /health
    OpenCodeServer-->>TmuxUtils: 200 OK
    
    TmuxUtils->>Shell: tmux split-window "opencode attach http://localhost:5000 --session <id>"
    Shell->>OpenCodeServer: Connect to session on port 5000
    OpenCodeServer-->>Shell: Stream session updates
    
    Note over TmuxUtils,Shell: Port from OPENCODE_PORT ensures<br/>correct server connection

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