core: restore non-experimental environment variable compatibility in v2

[opencode-agent]

Summary

V2 does not preserve a number of non-experimental environment-variable behaviors supported by V1. Some variables remain declared in packages/core/src/flag/flag.ts but have no active V2 consumer; others disappeared with their V1 subsystem.

This is the umbrella issue for the broader compatibility gap reported narrowly in #36837.

Audited against:

• V1 / dev: b6478dcebf1da4a559055c702c62b9294c41afdb
• V2: ba1f3d3d32690879fbed156d53570b8652e254ed

Completely ignored in V2

Configuration and permissions

• OPENCODE_CONFIG — explicit config-file override
• OPENCODE_CONFIG_CONTENT — inline config injection
• OPENCODE_PERMISSION — JSON permission override
• OPENCODE_DISABLE_AUTOCOMPACT — force-disable automatic compaction
• OPENCODE_DISABLE_PRUNE — force-disable compaction pruning
• OPENCODE_FAKE_VCS — override detected VCS

Sharing and authentication

• OPENCODE_AUTO_SHARE — force automatic sharing
• OPENCODE_DISABLE_SHARE — globally disable sharing
• OPENCODE_AUTH_CONTENT — in-memory auth JSON override

Plugins, skills, and compatibility discovery

• OPENCODE_PURE — suppress external plugins/runtime integrations
• OPENCODE_DISABLE_DEFAULT_PLUGINS — suppress built-in plugins
• OPENCODE_DISABLE_EXTERNAL_SKILLS — suppress external skill discovery
• OPENCODE_DISABLE_CLAUDE_CODE — suppress Claude compatibility discovery
• OPENCODE_DISABLE_CLAUDE_CODE_PROMPT — suppress Claude prompt discovery
• OPENCODE_DISABLE_CLAUDE_CODE_SKILLS — suppress Claude skill discovery
• OPENCODE_PLUGIN_META_FILE — override plugin metadata path

Other runtime controls

• OPENCODE_DISABLE_LSP_DOWNLOAD — prevent language-server installation
• OPENCODE_ENABLE_QUESTION_TOOL — control question-tool availability
• OPENCODE_DISABLE_EMBEDDED_WEB_UI — suppress embedded UI routes
• OPENCODE_AUTO_HEAP_SNAPSHOT — enable automatic heap snapshots
• OPENCODE_ALWAYS_NOTIFY_UPDATE — force update notifications

Partially handled or changed

• OPENCODE_DISABLE_PROJECT_CONFIG only suppresses project AGENTS.md. V2 still loads project opencode.json(c) and project .opencode, .claude, and .agents directories. See core: OPENCODE_DISABLE_PROJECT_CONFIG is ignored in v2 #36837.
• OPENCODE_CONFIG_DIR changes the global config root in V2 instead of acting as the additional explicit config source it was in V1. Confirm and restore the intended compatibility semantics.
• OPENCODE_SERVER_PASSWORD is still accepted, but the V2 CLI prefers OPENCODE_PASSWORD and managed-server credential propagation differs from V1.
• OPENCODE_SERVER_USERNAME only affects manually configured server authentication, not the new managed-service credential path.

Intentional change

OPENCODE_TUI_CONFIG is not included in the compatibility checklist. V2 intentionally moved to one global cli.json and does not support arbitrary or project-local TUI configuration.

Expected behavior

Per the V1-to-V2 migration contract, non-API V1 functionality is intended to remain compatible. For every item above, either:

1. restore the V1 behavior in V2 and add coverage, or
2. explicitly decide that the behavior is obsolete, remove stale declarations/documentation, and document the migration path.