SBX treats provider projection mount as a kit path #629

Problem

Real #601 validation fails before SBX sandbox creation after provider inputs validate successfully:

error: resolve kits: kit path .../provider-input/sbx/<name>.<id>/generations/generation.<id>:ro does not exist

The provider generation read-only mount is appended through the kit argument path, so SBX parses path:ro as a kit reference instead of a volume/mount.

Reproduce

macOS host, Quickstart validation worktree combining #611/#622/#619:

bash tmp/validate-provider-neutral-opencode.sh --repo tmp/validate-601-with-622 --backend sbx --apply

Evidence: agentic-coding-quickstart/tmp/provider-neutral-validation.aguo8ZPz/sbx.log.

Acceptance

• Provider input generation reaches the SBX guest read-only through the supported SBX mount mechanism, not kit resolution.
• ACQ_PROVIDER_INPUT_ROOT points to the mounted generation.
• Existing selected kits and workspace mounts retain order/semantics.
• Offline adapter tests assert exact SBX argv shape.
• Real Adopt provider-neutral OpenCode kits and verify both backends #601 SBX validation passes or advances to the next observed failure.

Introduced by provider projection work merged in #608.

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