feat: provider detection — ProviderSet.detect() probes environment

[justrach]

Parent: #257 — Layer 0 (Plumbing)

What

ProviderSet.detect(alloc) probes the environment and reports what's available. Detection only — no preferences, no routing decisions.

Probes:

• PATH for claude, codex binaries
• PATH for zig* tools (zigrep, zigread, zigpatch, etc.)
• PATH for fallback tools (rg, fd, git, sed)
• Loads config from .codedb/config.toml (repo) > ~/.config/devswarm/config.toml (global) > defaults

Returns:

const ProviderSet = struct {
    has_claude: bool,
    has_codex: bool,
    has_local: bool,          // future: ollama etc.
    config: ProviderConfig,   // parsed from config.toml
    tools: ToolAvailability,  // which file I/O tools are available
};

const ToolAvailability = struct {
    search: enum { zigrep, rg, grep },
    read: enum { zigread, cat },
    edit: enum { zigpatch, sed },
    diff: enum { zigdiff, git_diff, diff },
    find: enum { zigrep_f, fd, find },
    batch: enum { zigpar, sequential },
};

No preferredBackend() — that decision belongs to resolve().

Depends on

• feat: user config — .codedb/config.toml with [provider] + [agents] sections #266 (user config — to load .codedb/config.toml)

[justrach]

Blocked by: #262.

[justrach]

Blocks: #265, #266, #267, #268, #271, #272, #273