Canonicalize the artifact root (realpath) so isolated projects avoid symlinked temp paths

[kitlangton]

Drive 2.0.0 (464f9a3), OpenCode V2 0b771030ed and e73fa5f476.

Default artifacts live under os.tmpdir(), which on macOS is /var/folders/... → symlink to /private/var/folders/.... OpenCode currently discovers the isolated project's .opencode directory under both spellings, starts two watchers, loads every local plugin twice, and fails activation with Duplicate plugin ID. That is an OpenCode bug (tracked separately), but Drive is the thing putting every isolated project under a symlinked root, so every plugin probe on macOS hits it.

Proposal: resolve the artifact root with realpath in instance.ts (resolve(join(tmpdir(), "opencode-drive"))) and pass the canonical path everywhere (--dev, project files, OPENCODE_CONFIG_DIR). Until then the workaround is TMPDIR="$PWD/.drive-output/tmp", now documented in the tui-regressions README under "Plugin registry".

Repro: test/manual/tui-regressions/plugin-registry.ts without TMPDIR; server log shows two watcher started path=…/.opencode lines in one boot span followed by failed to reload plugins cause=Die(Error: Duplicate plugin ID: alpha).

[kitlangton]

Correction to the premise after the OpenCode-side fix landed (anomalyco/opencode#46841): a symlinked project alone did not reproduce the double discovery. The trigger was Drive pinning OPENCODE_CONFIG_DIR to <project>/.opencode while the isolated project sat under a symlinked temp root, so OpenCode's upward config walk reached its own global config dir under a different spelling and loaded it as both project and global config. OpenCode now compares those roots by realpath, so current V2 no longer hits it; the TMPDIR workaround in the tui-regressions README is obsolete against V2 ≥ 473c292521. Canonicalizing Drive's artifact root is still a reasonable hygiene change (fewer surprises for anything path-sensitive), but it is no longer needed to make plugin probes work.

[kitlangton]

There is still a location-scoping consequence in Drive 2.0.1 against OpenCode ac874a6e90: the generated SDK defaults to the /var/.../files spelling, while the TUI uses /private/var/.../files. A plugin RPC that changes captured transform state can fail the SDK-side plugin instance while the visible TUI still reports its separate instance as active.

Reproduced with a local fixture plugin registering a command transform and a demo.fail RPC that flips a boolean and calls command.reload(). The SDK inventory showed failed, but the TUI showed neither the notice nor the failed inventory row. Passing { location: { directory: realpathSync(artifacts + "/files") } } to the RPC and inventory reads made them target the TUI instance and the demo passed. Canonicalizing the artifact root remains useful beyond duplicate config discovery.