enhance(#3245): report the detected host runtime in init (#3307)

* test(#3245): failing-first coverage for host runtime detection in init

Locks the behavior epic #2313 Phase 5 must produce before any of it exists: init reports the detected host, explicit GSD_RUNTIME and config runtime still outrank detection, non-Codex sessions are untouched, and nothing is ever written to shared defaults (#2297).

* enhance(#3245): report the detected host runtime in init

init reported agent_runtime: claude inside a Codex session, and resolved agents_dir to the Claude agents root with agents_installed: true — a spuriously healthy triple. Runtime identity was only ever read from GSD_RUNTIME or an explicit runtime in .planning/config.json.

Adds a detection rung beneath both explicit sources, in a new pure module. Codex is identified from its own documented session environment (CODEX_SANDBOX / CODEX_SANDBOX_NETWORK_DISABLED), else an explicitly exported CODEX_HOME whose config.toml exists. The default ~/.codex is never probed: that file exists on every machine that has run Codex, so probing it would misreport other runtimes' sessions.

resolveRuntime keeps its exact contract and all 71 dependents, including formatGsdSlash command-style emission; only withProjectRoot consumes the new rung. Nothing is written on any path (#2297). Explicit config still wins (#2517).

* fix(#3245): make the parity guard real and single-source the marker

Four independent review passes found the generative-fix-divergence guard was vacuous: it asserted agreement at the one input where inferPreferredRuntime and detectHostRuntime do not differ, so it could not fail. It now pins the actual divergence point (CODEX_HOME set, config.toml absent) and records that the asymmetry is deliberate.

The config.toml marker is now single-sourced from update-context.cts and imported, rather than carried independently by two surfaces. tests/helpers.cjs now scrubs CODEX_SANDBOX and CODEX_SANDBOX_NETWORK_DISABLED: GSD reads them, so an ambient Codex session would otherwise make the non-codex control test fail non-deterministically.

Also: detection is throw-safe end to end rather than only around the fs probe; the Windows-join test is replaced with one that can actually fail (trailing-separator, catches hand-rolled concatenation); the #2297 no-write proof now wraps resolveReportedRuntime, the function that ships, across all three ladder outcomes.

* chore(#3245): backfill changeset pr number

---------

Co-authored-by: sim <sim@local>

docs/README.md

@@ -31,6 +31,7 @@ Language versions: [English](README.md) · [Português (pt-BR)](pt-BR/README.md)
 - [Run phases autonomously](how-to/run-phases-autonomously.md) — use autonomous mode for unattended phase execution
 - [Handle quick and fast tasks](how-to/handle-quick-and-fast-tasks.md) — use `/gsd-quick` and `/gsd-fast` for ad-hoc work outside the phase loop
 - [Configure model profiles](how-to/configure-model-profiles.md) — switch between quality, balanced, and budget model tiers
+- [Control which host runtime GSD reports](how-to/control-the-reported-host-runtime.md) — read the `agent_runtime` ladder, understand what host detection looks at, and pin the runtime when detection is not what you want
 - [Set up cross-AI review](how-to/set-up-cross-ai-review.md) — configure a second AI to review code produced by the primary agent
 - [Work in parallel with workstreams](how-to/work-in-parallel-with-workstreams.md) — run independent lines of work simultaneously using workstreams
 - [Isolate work with workspaces](how-to/isolate-work-with-workspaces.md) — use workspaces to sandbox experimental or risky changes