feat(installer): support Termux

[wahh-22]

Pre-flight Checks

• I have searched existing issues and this is not a duplicate
• I understand this issue needs status:approved before a PR can be opened

Problem Description

Termux users should be able to install tmux-agent-state with the same command used on macOS and Linux. Previously, missing dependencies such as jq required manual installation, and direct script execution could fail in Termux environments where /usr/bin/env is unavailable.

Proposed Solution

Detect Termux explicitly and automatically install missing Termux packages (tmux, jq, python) with pkg install -y ... before continuing. Keep non-Termux behavior unchanged: report missing dependencies and exit without attempting automatic package installation.

Affected Area

Scripts

Alternatives Considered

Documenting manual pkg install commands was considered, but that creates a different setup path for Termux users. The installer should preserve ./install.sh --all as the single happy path across supported environments.

Additional Context

The implementation was tested on Termux: the installer detected missing jq, installed it with pkg install -y jq, continued installation, and installed the expected tmux scripts and configuration.