feat(#1452): add workflow.context_guard_mode to guard execute-phase against context exhaustion

Proactive checkpoint guard fires at each wave boundary before spawning agents.
Self-assesses context pressure against context-budget.md degradation tiers and
warns (warn, default) or auto-invokes /gsd:pause-work (auto) when POOR tier
(70%+) is detected. Config key validated; defaults to \"warn\".

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

gsd-core/references/planning-config.md

@@ -267,6 +267,7 @@ Set via `workflow.*` namespace in config.json (e.g., `"workflow": { "research":
 | `workflow.test_command` | string\|null | `null` | Any shell command | Regression/test gate command run by verify-phase, execute-phase, audit-fix, and post-merge-gate. Unset → GSD auto-detects (Makefile / package.json / Cargo.toml / go.mod / pyproject.toml). |
 | `workflow.build_command` | string\|null | `null` | Any shell command | Build gate command run by the post-merge gate. Unset → build step auto-detected/skipped. |
 | `workflow.mvp_mode` | boolean | `false` | `true`, `false` | Persist the MVP-mode flag in config so every phase defaults to MVP framing without requiring `--mvp` on the CLI. Resolved via the chain: `--mvp` CLI flag → ROADMAP.md `**Mode:** mvp` field → this config value → `false`. When `true`, the planner, executor, verifier, and discovery surfaces (progress, stats, graphify) all treat the phase as an MVP vertical slice (UI → API → DB) of one user-visible capability. |
+| `workflow.context_guard_mode` | string | `"warn"` | `"auto"`, `"warn"`, `"off"` | Context exhaustion guard mode for `execute-phase`. Before each wave, the orchestrator self-assesses context pressure using degradation signals from `context-budget.md`. `"warn"` (default): emit a warning and recommend `/gsd:pause-work` when POOR tier is detected. `"auto"`: automatically invoke `/gsd:pause-work` before the next wave when POOR tier is detected. `"off"`: disable the guard. The guard is heuristic — no programmatic context-% API exists. |
 | `workflow.plan_chunked` | boolean | `false` | `true`, `false` | Enable chunked planning mode. When `true`, the plan-phase orchestrator splits the single long-lived planner Task into a short outline Task followed by N short per-plan Tasks (~3–5 min each). Each plan is committed individually for crash resilience. Particularly useful on Windows where long-lived Tasks may hang on stdio. Also activated by the `--chunked` flag. |
 | `workflow.code_review_command` | string\|null | `null` | Any shell command | External code-review command integrated into `/gsd:ship`. The diff is piped to the command via stdin; the command must output JSON with a `verdict` field (`"APPROVED"` or `"REVISE"`). Non-zero exit or `"REVISE"` verdict blocks the ship workflow. When unset, the built-in review flow runs. Example: `my-review-tool --review`. |
 | `workflow.inline_plan_threshold` | number | `2` | `0`–`10` | Plans with ≤N tasks execute inline instead of spawning a subagent |