P2: Prompt-ingest: a docs page and a runnable example

[macanderson]

Follow-up to #44 / ADR 0006. The contextgraph_host::ingest provider is covered by unit + integration + conformance tests, but has no user-facing docs page or runnable example — the module doc comment and the ADR are the only prose.

• docs/prompt-ingestion.md — the concept (paste → intent + anchors + content-addressed evidence frames), the compact-by-default / pull-[full]-on-demand model, and how it composes byte-stably through compose_context. Sits alongside docs/implementing-a-provider.md and docs/context-reuse.md.
• A runnable example — a small examples/-style program (or a contextgraph-inspect mode) that takes a paste, prints the SegmentReport pills, registers the IngestProvider in a Host, fans out, and prints the composed block. Makes the "pills are visible and correctable" UX guarantee concrete.

Nice-to-have: a golden NDJSON transcript of an ingest frames envelope added to examples/ so the existing schema/validate-examples.py CI step covers a real ingested frame directly (complements the Rust conformance test from #44, and relates to #54).

[macanderson]

Re-verified 2026-07-26 against origin/main @ 1ee9b1c.

There is no prompt-ingestion.md under docs/. examples/ holds three files and no ingest transcript, so the schema CI job currently validates no ingested frame at all — the ingest path has documentation-shaped prose but zero machine-checked examples. The only ingest prose in the repo is ADR 0006 and a module doc comment.

Recommended scope: keep. Promote the golden NDJSON from nice-to-have to a checkbox — it is the only part of this issue with a CI consequence, and it overlaps #54's "validate serialized reference values" work, so doing them together gets the ingest path its first real schema witness.