Unify integration installer interface (is_installed / install_state)

[Cannon07]

Motivation

Each integration (lua/code-preview/backends/{claudecode,copilot,codex,opencode}.lua) answers "is this installed?" with a different shape:

• claudecode — inline string search in init.lua's status() for "code-preview" / "claude-preview" in .claude/settings.local.json. No module function.
• copilot — is_our_config(path) boolean.
• codex — is_installed() plus feature_flag_state() ("enabled" / "disabled" / nil), composed in status().
• opencode — file-existence check inlined in status().

Adding a fifth agent means inventing a fifth pattern. status() is the only consumer and is unnecessarily aware of each integration's internals.

Proposed direction

A shared shape on every integration module:

--- @return { state: "installed" | "missing", warnings: string[]? }
function M.install_state() ... end

Where warnings is for cases like Codex's missing codex_hooks flag — installed but degraded. init.lua's status() becomes a loop over a registry of integrations.

Scope

• Add M.install_state() to all four integration modules.
• Define a registry (probably in init.lua or a new backends/init.lua) so status() and future code can iterate.
• Move the inline claudecode/opencode detection out of init.lua into their respective modules.
• No behaviour change for the user; :CodePreviewStatus output should look identical.

Out of scope

• Adding new agents.
• Changing the install / uninstall command shape.