feat: add missing features from spec

[ZeeshanAdilButt]

Summary

Implements all practical missing features from specs/OLOSTEP_CLI_SPEC.md that were marked [ASPIRATIONAL] or [PARTIAL] but feasible for this release.

New commands

• olostep search <query> — web search via POST /v1/searches. Flags: --limit, --include-domains, --exclude-domains, --json, --out
• olostep auth subcommand group — auth login, auth logout, auth status, auth set-key <key>, auth set-token <token>
• olostep doctor — health checks: auth presence, API reachability, MCP endpoint, per-agent config existence. Flags: --json (NDJSON), --skip-network, --fail-on-warn
• olostep version — structured version output. Flags: --json
• olostep skills subcommand group — skills install, skills list, skills uninstall, skills update

New MCP clients

Added 4 missing install targets to mcp install/mcp uninstall/list mcp:

• claude-desktop — platform-aware config path (macOS/Linux/Windows)
• opencode — global + project-scoped config
• continue — array format for mcpServers (not object map)
• codex — TOML format (inline serializer, no new deps)

Env vars

• OLOSTEP_JSON — global JSON mode (equivalent to passing --json on every command)
• OLOSTEP_NO_UPDATE_CHECK — alias for OLOSTEP_NO_UPDATE_NOTICE (both supported)

Test plan

• npm test — 167 tests, 11 suites, all passing
• npm run typecheck — clean
• olostep search "test query" — returns numbered results
• olostep auth set-key sk_xxx — writes credentials.json
• olostep doctor --skip-network — shows auth + config checks only
• olostep version --json — returns { cli, node, channel }
• olostep mcp install --agent claude-desktop — writes correct config path
• OLOSTEP_JSON=1 olostep status — outputs JSON without --json flag