feat: add custom binary config for gemini, opencode, codex tools

[liqimore]

Background

Previously, only Claude supported custom binary configuration via [claude].command in config.toml. Gemini, OpenCode, and Codex all had their binary names hardcoded in the respective
buildXxxCommand() functions. This meant users who needed to launch these tools through a wrapper or proxy binary (e.g. codex-proxy) had no supported way to configure this — and session
resume/restart logic would break when a custom command name was in use.

Changes

Add a command = "..." config field to [gemini], [opencode], and [codex] sections in config.toml, following the exact same pattern as Claude.

internal/session/userconfig.go

• Add Command string \toml:"command"`toGeminiSettings, OpenCodeSettings, CodexSettings`

internal/session/gemini.go

• Add GetGeminiCommand() — returns configured binary or defaults to "gemini"

internal/session/instance.go

• Add GetOpenCodeCommand() — returns configured binary or defaults to "opencode"
• Add GetCodexCommand() — returns configured binary or defaults to "codex"
• Update buildGeminiCommand, buildOpenCodeCommand, buildCodexCommand to use the getter; isStandardStart now matches either the canonical name or the configured binary, so session
  resume logic is triggered correctly in both cases

Usage

[codex]
command = "codex-proxy"   # replaces "codex" everywhere, including resume

[gemini]
command = "gemini-proxy"

[opencode]
command = "opencode-nightly"

note:
claude-proxy means the binary was start using proxychains4, eg:

% cat ~/bin/claude-proxy
#!/bin/bash
if ! ss -tlnp 2>/dev/null | grep -q ':1080'; then
    echo "   → sudo systemctl start shadowsocks-libev-local@claude-proxy"
    exit 1
fi

unset http_proxy https_proxy HTTP_PROXY HTTPS_PROXY
unset no_proxy NO_PROXY ALL_PROXY all_proxy

exec proxychains4 claude "$@"

[johnrichardrinehart]

Please see #550 as it's closely-related.

[liqimore]

Please see #550 as it's closely-related.

Thanks for point out, #550 would be a more sutable solution.