discovery: env-var auto-injection following LSH_<SERVICE>_<PROP>

[pierrick-fonquerne]

Expose each resource connection information to the others through environment variables, so a polyglot developer never wires URLs by hand.

Scope

• Naming convention LSH_<SERVICE>_<PROPERTY> documented and enforced (uppercase, snake case input normalised).
• Resolution of ${resources.<name>.<property>} against the runtime outputs of the dependency.
• Integration with the lifecycle manager so dependents observe the variables of their dependencies before starting.
• Escape semantics for literal ${ in user-provided values.

Acceptance criteria

• Spec section in docs/spec/manifest-v0.md describes the convention with examples.
• Round-trip integration test: a container resource reads LSH_POSTGRES_URL produced by a postgres resource and prints it to stdout.
• Invalid property paths produce errors at validation time, not runtime.