feat(installer): add Claude Desktop and OpenCode, auto-configure by default

[antonisobkowicz-asana]

Summary

• Claude Desktop: detected via its app bundle (not a CLI command, unlike every other client here), configured by writing ~/Library/Application Support/Claude/claude_desktop_config.json in the same mcpServers shape Claude Code/Cursor use (minus the type field, which Claude Desktop infers from the presence of command).
• OpenCode: detected via the opencode command, configured by writing ~/.config/opencode/opencode.json under a distinct mcp key with its own entry shape (type: "local", and command as a single array combining the executable and its arguments, rather than separate command/args fields).
• ChatGPT Desktop: investigated and intentionally not given its own flag — it shares Codex CLI's ~/.codex/config.toml on the same host, so --codex already covers it when the codex command is installed. Documented this relationship rather than adding a redundant path. (A narrower gap exists for users with ChatGPT Desktop but no codex CLI binary; left out of scope for this PR since it would need a hand-rolled TOML writer for a comparatively rare case.)
• Extracted the JSON read-merge-write logic now shared by Cursor, Claude Desktop, and OpenCode into one write_mcp_json_config shell function parameterized by config path, top-level servers key, and entry shape — added here since a third near-identical 40-line block crossed the threshold where duplicating it again stopped being the simpler option. Cursor's own output is byte-for-byte unchanged.
• Extended the legacy-package-reference scan (the logic that offers to delete old manually-installed .tgz files after a config gets rewritten) to also read both new config files.

Behavior change: auto-select by default

While implementing this, hit a real question: should --all (and the true no-flags default) die if one of five possible clients isn't installed? That would make the common case — most people have one or two of these, not all five — fail by default. So:

• No flags, or --all: auto-detect and configure every client found; silently skip ones that aren't. No interactive Y/n prompting either — retired the prompt_target confirmation step entirely, since the piped curl | sh install path already always ran in exactly this way when non-interactive (no TTY), it just also happened to prompt when run interactively in a terminal. Now both cases behave the same: install what's there.
• An explicit single-client flag (--claude, --claude-desktop, --codex, --cursor, --opencode): unchanged from today — requires exactly that client and fails loudly if it's missing.

Test plan

• sh -n install.sh
• npm run typecheck
• npm run lint
• npm test (373 tests passing — new coverage for Claude Desktop and OpenCode configuration, the auto-select-skips-undetected-clients default, explicit-flag-dies-when-missing for both new clients, and legacy-package cleanup scanning both new config files)
• npm run build