[FEATURE] ERL system-prompt injection of top-k heuristics (Child C of #1303)

[Lexus2016]

Parent: #1303

Slice C of the ERL-style cross-task heuristic extraction pipeline.

Scope (~80 lines)

Inject top-k heuristics into the system prompt at conversation start:

• Read ranked heuristics from Slice B
• Format as a concise "Learned Heuristics" block in the system prompt
• Gate behind config (evolution.erl_prompt_injection: true)

Dependencies

• Requires Slice B (#child-b) to be merged first

Acceptance Criteria

• Heuristic block appears in system prompt when enabled
• Block is cache-safe (stable for the conversation lifetime)
• Tests verify prompt formatting and config gating

Merge cap

≤ 200 lines total.

[Lexus2016]

Blocked: Child C (#1361) depends on Child B (#1360) which is itself blocked on Child A (#1359 / PR #1397). Cannot implement until the ERL pipeline foundation lands on main.