docs: add config.md reference for the daemon TOML schema

README previously punted on the config schema with a "full key list in
internal/config/config.go" pointer. New docs/config.md walks every
TOML key the daemon reads — top-level, [vm_defaults], [[file_sync]] —
with type, default, and a one-sentence description per row, plus a
copy-pasteable example at the bottom.

Sourced 1:1 from internal/config/config.go's fileConfig (and the
defaults in load() + internal/model/types.go), so it stays accurate
as long as those structs are the schema source of truth.

README's existing config section now points at docs/config.md, and
the "Further reading" list gets it as the first bullet.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -211,7 +211,7 @@ Most commonly set:
   paths are rejected at config load.
 - `firecracker_bin` — override the auto-resolved `PATH` lookup.
 
-Full key list in `internal/config/config.go`.
+Full key reference: [`docs/config.md`](docs/config.md).
 
 ### `vm_defaults` — sizing for new VMs
 
@@ -293,6 +293,7 @@ guest IPs to an untrusted network.
 
 ## Further reading
 
+- [`docs/config.md`](docs/config.md) — full config key reference.
 - [`docs/dns-routing.md`](docs/dns-routing.md) — resolving
   `<vm>.vm` hostnames from the host.
 - [`docs/image-catalog.md`](docs/image-catalog.md) — bundle format