docs: ADR 0001-0003 — OpenCode commands, config merge, permissions default

Why: Architectural decisions recorded during planning phase.
See docs/plans/feature_opencode-commands_as_md_and_config_merge.md for full context.

docs/decisions/0002-opencode-json-merge-strategy.md

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+# ADR 0002: Plugin merges into existing opencode.json rather than replacing it
+
+## Status
+Accepted
+
+## Date
+2026-02-20
+
+## Context
+Users have existing opencode.json files with personal configuration. The install command previously backed up and replaced this file entirely, destroying user settings.
+
+## Decision
+writeOpenCodeBundle reads existing opencode.json (if present), deep-merges plugin-provided keys without overwriting user-set values, and writes the merged result. User keys always win on conflict.
+
+## Consequences
+- Positive: User config preserved across installs. Re-installs are idempotent for user-set values.
+- Negative: Plugin cannot remove or update an MCP server entry if the user already has one with the same name.
+- Neutral: Backup of pre-merge file is still created for safety.
+
+## Plan Reference
+Originated from: docs/plans/feature_opencode-commands_as_md_and_config_merge.md