Add migration helper for Tabnine CLI to opencode - #1

Summary

Adds a migration_helper/ subtree that helps users move their Tabnine CLI (or Gemini CLI) configuration into opencode.

It contains two opencode skills that run as interactive wizards inside opencode itself, plus slash commands and a small bash installer:

• migrate-from-tabnine-cli (/migrate) — migrates MCP servers, skills, subagents, slash commands, and the contents of Tabnine CLI extensions. Run once per target scope (global or project).
• migrate-tabnine-context (/migrate-context) — migrates context/memory files (TABNINE.md, GEMINI.md, or custom context.fileName files) into opencode's AGENTS.md. Re-runnable per repository.

What's inside migration_helper/

migration_helper/
├── LICENSE                       # MIT, subtree-scoped
├── README.md                     # install (script or manual) + usage
├── install.sh                    # bash installer (executable)
├── commands/
│   ├── migrate.md                # /migrate slash command
│   └── migrate-context.md        # /migrate-context slash command
└── skills/
    ├── migrate-from-tabnine-cli/
    │   ├── SKILL.md              # main wizard skill
    │   └── references/
    │       ├── source-map.md     # where Tabnine CLI stores each thing
    │       └── mapping.md        # field-by-field translation rules
    └── migrate-tabnine-context/
        └── SKILL.md              # context-file wizard skill

What it migrates

MCP servers (including field renames opencode requires, e.g. env → environment, and $VAR → {env:VAR} value rewrites), skills, subagents, slash commands, extension contents, and context files. Fields with no opencode equivalent are dropped with a note in the wizard's summary. Nothing is deleted from the Tabnine CLI installation — both wizards are copy-and-translate flows, and neither overwrites an existing opencode file without asking.

Out of scope: OAuth tokens (users re-authenticate remote MCP servers on first use), Tabnine credentials, admin policy fields, hooks, themes, keybindings, and general settings.

Installer behaviour

./install.sh                 # global: ~/.config/opencode/
./install.sh --project       # project: ./.opencode/
./install.sh --overwrite     # overwrite existing files without prompting

For every file it wants to place:

• Missing target → copy.
• Identical target → skip (idempotent).
• Different target → show a unified diff, prompt [s]kip / [o]verwrite / [r]ename (rename appends .bak-YYYYMMDD-HHMMSS).
• No terminal to prompt on (e.g. piped/CI runs) → keep the existing file and say so, never overwrite.

Manual cp commands are documented as a fully supported alternative (recommended path on Windows).

Verification

• bash -n install.sh passes; installer tested on macOS (system bash 3.2) and Debian: fresh install (6 files), idempotent re-run, --project, --overwrite, interactive skip/overwrite/rename, and non-interactive runs.
• Both wizards exercised end-to-end inside opencode against sandboxed fixture homes covering: remote/local MCP servers (field renames, enablement, disabled + stale entries), skills (including name normalization and Gemini-specific-body flagging), agents (field translation, remote A2A skip), commands (all placeholder rewrites, nested namespacing), extension unpacking, Gemini-mode layout (~/.gemini), and context-file migration (fresh target, merge into existing AGENTS.md, repeat runs).
• Migrated configs verified to boot opencode cleanly with the items registered; source trees verified byte-identical after every run.
• Reference docs (source-map.md, mapping.md) verified against the Tabnine CLI and opencode sources.

Root README

One new section, ## Migrate from Tabnine CLI, links to migration_helper/README.md. Otherwise the root README is unchanged.

Licensing

migration_helper/LICENSE scopes this subtree to MIT with Copyright (c) 2026 Tabnine Ltd.. This is a documentation + shell subtree; MIT is the shortest, most permissive, best-known license for that shape. The root repo's proprietary posture (© Tabnine. All rights reserved.) is unchanged — only this subtree is MIT.