feat(prompts): support custom stages, remote prompts, and template variables

Add StagesConfig and StageDefinition for TOML-based stage
definitions. Add PromptRegistry::with_settings async factory
for remote prompt resolution (HTTP, git clone, local dir).
Add substitute_variables for {{variable}} template expansion.
Refactor get_stage_prompt to use fallback chain: stages.toml
config -> file-based stages -> hardcoded stages. Add stage
filtering respecting enabled flag from config. Add md5
dependency for prompt cache hashing.

Signed-off-by: derekbarbosa <derekasobrab@gmail.com>

Author: derekbarbosa

Cargo.lock

@@ -53,6 +53,7 @@ ignore = "0.4.25"
 lettre = { version = "0.11.7", features = ["tokio1", "tokio1-native-tls"] }
 libsql = "0.9.29"
 mail-parser = "0.11.1"
+md5 = "0.7.0"
 regex = "1.12.2"
 reqwest = { version = "0.13.1", features = ["json"] }
 serde = { version = "1.0.228", features = ["derive"] }