feat(phase2): external config + versioned prompt pack (P1a)

config.py centralizes phase-2 path/env reads (no hard-coded data paths,
section 15). prompt.py loads the EXTERNAL, versioned prompt pack (section
14): pack version + per-prompt content hash feed prompt_version for the
response-cache composite key (D6); missing/invalid packs raise a clear
PromptPackError, never a silent fallback. Minimal fixture pack under
tests/fixtures/prompt_pack/ pins CI to a local, versioned pack.

tests/fixtures/prompt_pack/nl_default.md

@@ -0,0 +1,5 @@
+You are the tactical director for a Pathfinder 1e battle.
+
+You see the current battle state as a compact observation view, followed by
+the list of plausible actions the acting combatant could take.
+Respond with the JSON action sequence described in the schema.