fix(consolidation): escape literal braces in caller-supplied mission text

[cdbartholomew]

Summary

The batch consolidation prompt is assembled from caller-supplied text (observations_mission, observation_capacity_note) interleaved with literal {facts_text} / {observations_text} placeholders, then passed through str.format() at call time. Any { or } in the caller-supplied text was previously interpreted as a format placeholder and raised KeyError, taking down consolidation for that bank — for example, a mission containing a JSON example like {"key": value} would crash with KeyError: "'key'".

This change escapes lone braces before assembly so the rendered prompt contains the original characters verbatim.

The escape is idempotent: text that already contains {{ / }} pairs is left untouched, so callers that pre-escape (or future call sites that defensively double-escape) still produce the same rendered output.

Implementation

• prompts.py adds _escape_braces() which doubles only lone { and } characters, leaving already-doubled pairs alone.
• Both observations_mission and observation_capacity_note are routed through it before being f-string-interpolated into the assembled prompt.
• Real format placeholders ({facts_text}, {observations_text}) live in module-level constants that already use {{ / }} for any literal JSON in the prompt, so they are unaffected.

Tests

17 new tests in tests/test_consolidation_prompt_brace_escape.py:

• _escape_braces unit coverage (lone brace doubling, idempotency, no-op on plain prose, mixed lone + already-escaped input).
• End-to-end coverage on build_batch_consolidation_prompt:
  • JSON-shaped mission text renders without crashing and appears verbatim.
  • Pre-escaped mission renders to single literal braces (no double-escape artefacts).
  • Multiple brace pairs in a single mission.
  • Capacity note containing braces.
  • The unchanged {facts_text} / {observations_text} substitution path still works.
  • Parametrised edge cases: {single}, }}weird{{, trailing {, leading }, empty string.

Test plan

• uv run pytest tests/test_consolidation_prompt_brace_escape.py -v → 17 passed
• uv run pytest tests/test_consolidation_failure_recovery.py tests/test_consolidation_prompt_brace_escape.py tests/test_format_facts_for_prompt.py → 30 passed (pre-existing collection errors in test_consolidation_round_limit.py reproduce on clean origin/main and are unrelated)
• uv run ruff check clean
• uv run ty check hindsight_api/engine/consolidation/prompts.py clean
• CI green on this PR before merge