[FEATURE]: Support OPENCODE_CACHE_DIR and OPENCODE_LOG_DIR for full isolation

[Robitx]

Feature hasn't been suggested before.

• [x] I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

Following up on issue #4526 and PR #8963. While moving the data directory is a great start, OpenCode still writes logs, state, and transient caches to fixed global system paths. This makes it difficult to run truly isolated instances.

Proposed Solution Add support for:

• OPENCODE_CACHE_DIR: For all transient state (indexes, caches, npm plugins).
• OPENCODE_LOG_DIR: For session logs.
• OPENCODE_STATE_DIR: For UI state (model selection, prompt history, stashed prompts, frecency data).

Why?

• Environment Hygiene: Prevents metadata leakage between different project contexts.
• Portability: Allows a user to wrap an entire environment (config, data, cache, logs, state) into a single path for easier backup or migration.
• Multi-instance Support: Running multiple OpenCode sessions with different identities/configs without path collisions in ~/.cache, /.local/share, or /.local/state.

EDIT:

I've extended PR #8963 to cover this also. The PR now includes all four environment variables:

• OPENCODE_DATA_DIR (original request from #4526)
• OPENCODE_CACHE_DIR (added to address this issue)
• OPENCODE_LOG_DIR (added to address this issue)
• OPENCODE_STATE_DIR (added to address this issue)

With all five environment variables (OPENCODE_CONFIG_DIR, OPENCODE_DATA_DIR, OPENCODE_CACHE_DIR, OPENCODE_LOG_DIR, OPENCODE_STATE_DIR), OpenCode achieves full portable mode with zero reliance on default XDG directories.