fix(rag): include heading_path in the prompt context blocks

[n0nuser]

Closes #172

Problem

build_prompt read only source and chunk_index off each context and never touched context["metadata"], so heading_path reached the API sources list (engine.py:214) but never reached the model.

The structural chunker computes heading_path precisely so a chunk carries its position in the document hierarchy. Discarding it at the prompt boundary threw away the one signal distinguishing a passage about long-term epidemiological risk from one about immediate effects — which is how a query about a single short night got answered with chronic-exposure statistics.

The metadata was already there: the compressor copies the whole dict (compressor.py:105, popping only expanded_text), so this is a missing read, not missing plumbing.

Change

Context block headers gain a section= segment when heading_path is populated:

[1] source=book.md chunk=473 section=Cancer, Heart Attacks > SLEEP LOSS AND THE CARDIOVASCULAR SYSTEM

The segment is omitted entirely when heading_path is empty, missing, or metadata is absent, so text_block/code_block chunks and the ollama.py call site (which passes no metadata) keep the exact previous header.

On the truncation bound

The issue flagged token cost as worth checking. It was: compression budgets measure only chunk body text and reserve nothing for block scaffolding, so an unbounded heading path grows the prompt past what any budget counted.

reserved_prompt_tokens (512) is the reservation covering everything outside chunk bodies. Measured worst case — max_contexts headings of single-character words, one whitespace token per two characters — comes to 518 tokens at a 200-char bound and 262 at 120. MAX_SECTION_CHARS is therefore 120, and a test pins the invariant rather than leaving it as a comment that drifts.

Verification

• Unit suite: 486 passed, 0 failed (6 new tests).
• Lint, format, mypy: clean.
• Integration, against a rebuilt image: 13 passed.
• End-to-end against real Chroma (4,215 ingested chunks): section= present on all 4,043 chunks carrying heading_path, absent on the 172 without.

Notes for the reviewer

Two pre-existing issues surfaced during verification, both out of scope here:

• Taskfile.yml:127 — LOCALRAG_BUILD_SHA="$$(git rev-parse HEAD)" does not expand outside Task's own escaping, so /build-info reports the literal string 99848(git rev-parse HEAD) and task docker-check can never pass.
• The unit suite reads a developer's local .env, so 26 tests fail on a machine that has one. Moving .env aside takes the suite from 26 failed to 0.

https://claude.ai/code/session_01PhFVTFLaxhmP8icwzmcm5Q