[refactor] Decompose ContextManager to meet ≤300-line module guideline

[dgenio]

Context

src/contextweaver/context/manager.py is currently 724 lines (was 642 before #100). The project guideline targets ≤300 lines per module (except __main__.py). This has been a pre-existing issue that grew further with the progressive disclosure feature.

Raised during review of #100 — extracting just the drilldown methods (~37 lines) wouldn't meaningfully address the problem, so a broader decomposition is needed.

Current responsibilities in ContextManager

Section	Approx lines	Candidate module
Ingestion helpers (ingest_tool_result, ingest_mcp_result, add_context_item)	~120	context/ingest.py
Episodic memory (add_episode, add_episode_sync)	~40	context/episodic.py or inline in store
Drilldown (drilldown, drilldown_sync)	~65	context/drilldown.py
Core pipeline (_build, build, build_sync)	~200	stays in manager.py
Properties + __init__	~50	stays in manager.py

Acceptance criteria

• manager.py ≤ 300 lines (or as close as practical)
• Extracted modules are self-contained with clear interfaces
• No public API changes — ContextManager still exposes all current methods
• All existing tests pass without modification (or with minimal import-path updates)
• make ci passes

Constraints

• Zero runtime dependencies
• Extracted helpers may receive manager internals as parameters (artifact_store, event_log, estimator, etc.)
• Prefer composition over mixins — keep inheritance simple
• Each new module ≤ 300 lines

Related

• feat: progressive disclosure for tool results — view registry + drilldown loop #100 — PR that prompted this discussion
• [context] Progressive disclosure for tool results — view registry + drilldown loop #17 — progressive disclosure feature that added drilldown methods