docs(#1672): add ADR-1671 dynamic context management platform + reference example (#1674)

Phase 0 of the Dynamic Context Management epic (#1671): the ADR (documentation) plus a non-shipping reference example under examples/dynamic-context-management/ — the Option-E predicate fact-store parser/selector, a self-contained --check/--write/--select index generator, a sample index, and a runnable demo.

The example is intentionally excluded from the build (src/->bin/lib/), the npm package files[], the installer, and the CI test suite (tests/) — nothing here is compiled into or installed with GSD. Production implementation lands in a later phase.

Resolves #1672

Refs #1671

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

examples/dynamic-context-management/README.md

@@ -0,0 +1,44 @@
+# Dynamic context management — Option-E reference example
+
+Reference example for [ADR-1671](../../docs/adr/1671-dynamic-context-management-platform.md),
+"Dynamic context management platform."
+
+> **This is a non-shipping reference example.** It lives outside the build
+> (`src/` → `bin/lib/`), the npm package `files[]`, the installer, and the CI
+> test suite (`tests/`). Nothing here is compiled into or installed with GSD.
+> The production implementation lands in a later phase of the
+> [Dynamic Context Management epic (#1671)](https://github.com/open-gsd/gsd-core/issues/1671).
+
+## What it demonstrates
+
+The **predicate fact-store → JIT selector** slice of the platform: parse the
+repo-root `CONTEXT.md` `CLASS.subkey=value` predicates into structured records,
+drift-guard a generated index, and select the relevant predicate subset for a
+task — the building block for just-in-time agent-brief assembly instead of
+hand-citing a 200 KB file.
+
+## Files
+
+- `context-predicates.cjs` — parser + selector + deterministic index builder (self-contained).
+- `gen-context-index.cjs` — `--check` / `--write` drift-guarded generator + `--select`.
+- `CONTEXT-INDEX.json` — sample generated output (393 predicates, 18 classes).
+- `demo.cjs` — runnable usage example.
+
+## Run (from the repo root)
+
+```sh
+node examples/dynamic-context-management/demo.cjs
+node examples/dynamic-context-management/gen-context-index.cjs --select PRED.k320
+node examples/dynamic-context-management/gen-context-index.cjs --check
+```
+
+## Validation
+
+During research this slice was validated with 42 behavioral tests — predicate
+forms, fenced-code / prose skipping, duplicate-id detection, the selector, a
+deterministic index, and a fast-check property test. Those return as CI tests
+under `tests/` when the production implementation lands.
+
+It also surfaced 3 latent duplicate predicate IDs in `CONTEXT.md`
+(`RULESET.WORKFLOW_MARKDOWN.FENCES`, `RULESET.GEMINI.TOOLS.ask_user`,
+`RULESET.GEMINI.TEST_SENTINEL`), recorded in the index `duplicates` field.