fix(desktop): remove dead experimental env setters - #321

Problem

Fixes #115. The desktop app set OPENCODE_EXPERIMENTAL_ICON_DISCOVERY="true" and OPENCODE_EXPERIMENTAL_FILEWATCHER="true" into its environment on every launch (packages/desktop/src/main/lifecycle/environment.ts), intending to enable icon discovery and the file watcher for the servers it spawns — but nothing in the codebase reads either variable, and the English docs (cli.mdx Experimental table) promised both to users as working enable switches.

• File watcher: already on by default and governed by different names — packages/cli/src/server-process.ts computes filewatcher: !truthy(process.env.OPENCODE_FILEWATCHER_DISABLE ?? process.env.OPENCODE_DISABLE_FILEWATCHER). Setting OPENCODE_EXPERIMENTAL_FILEWATCHER=true could never change anything.
• Icon discovery: no such feature exists in v2 under any name (repo-wide search finds no reader and no feature outside the setter itself).

Solution

Direction 2 from the issue (remove the dead setters, align the docs), following the #103 precedent where the docs/setters were aligned with deliberate v2 behaviour:

• environment.ts: drop both entries from preferApplicationEnvironment; OPENCODE_CLIENT: "desktop" is unchanged. This is behaviour-neutral — the variables had no reader, so no spawned server can observe a difference.
• cli.mdx (English source): remove the OPENCODE_EXPERIMENTAL_ICON_DISCOVERY and OPENCODE_EXPERIMENTAL_FILEWATCHER rows from the Experimental table. Translations follow the usual docs-locale-sync flow, per the docs: OPENCODE_SERVER_USERNAME is promised but never read — basic-auth username is hardcoded to 'opencode' #103/docs: troubleshooting.mdx says the desktop app uses OPENCODE_PORT for its local server — nothing reads it #117 precedent.

After this change, no launch-time setter in the desktop lifecycle names a variable nothing reads, and the docs no longer list either variable as a working enable switch — the acceptance criteria of #115. No test is added: the desktop package's testing rules prohibit source-grep/tombstone tests, and there is no behaviour to pin (the removed assignments were unobservable).

Verification

• git grep for both names across the tree, excluding locale docs: no matches remain (setter and English rows gone).
• Prettier check passes on both changed files.
• Desktop package typecheck run locally (see PR checks for CI).

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