Add explicit hosted OpenCode command

[sanchitmonga22]

Summary

Adds the explicit hosted OpenCode command on top of the Cloud auth branch.

What changed

• Adds rcli opencode --cloud --model <console-model-id> with -- argument passthrough.
• Executes the installed OpenCode binary directly and injects an ephemeral OPENCODE_CONFIG_CONTENT.
• Restores the caller environment after exit and avoids persistent OpenCode/tool-config mutations.
• Refreshes the session when needed and never prints credentials.

Branch/base

This PR is intentionally stacked on frontier/auth-device-flow and should merge after the auth PR. It is not a replacement for the auth implementation.

API/contract notes

The command consumes the public model identifier and bearer-backed Cloud session established by the auth branch. It does not add automatic model routing or billing behavior.

Validation

• Official kit build passed.
• CTest: 7/7 passed.
• Focused OpenCode tests: 3/3 passed.
• Installed OpenCode 1.1.53 help probe passed.
• Fake loopback Cloud session returned runanywhere/frontier/test-model; no fake token persisted.

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[sanchitmonga22]

Superseded by stacked follow-up PR #50, which preserves Siddhesh's #34 harness/editor work and carries the hosted OpenCode change together with hardened account transport. Closing the duplicate branch so there is one review path.

[Siddhesh2377]

Overlap worth checking: PR #34 also adds an opencode command, and both touch src/app.cpp, src/commands/commands.h and the CMake wiring.

One implementation detail from #34 that may be relevant here: the provider config is passed to the spawned process through the OPENCODE_CONFIG_CONTENT environment variable and restored afterwards, rather than written to a file. Writing to the user's project or to ~/.config/opencode outlives the session and changes how opencode behaves when they run it themselves, which is a surprising thing for a CLI to do.

Not asking for a change — just flagging it so whoever merges second knows what they are reconciling against.