ci: bump dev-env image to v1.2.0

Bump the pinned icp-dev-env image from 1.1.0 to the v-prefixed v1.2.0
tag in the two source-of-truth locations, and document the v-prefix
convention in AGENTS.md.

The registry publishes both `v1.2.0` and `1.2.0` tags (from
type=ref,event=tag and type=semver respectively); we pin the
v-prefixed form. dev-env-version-check.yml keeps devcontainer.json and
_run-example.yml in sync.

Awaiting dfinity/icp-dev-env#43 (the v1.2.0 release); CI will be red
until that image is published.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: marc0olo

.devcontainer/devcontainer.json

@@ -1,6 +1,6 @@
 {
   "name": "ICP Examples (Motoko + Rust)",
-  "image": "ghcr.io/dfinity/icp-dev-env-all:1.1.0",
+  "image": "ghcr.io/dfinity/icp-dev-env-all:v1.2.0",
   "forwardPorts": [8000, 5173],
   "portsAttributes": {
     "8000": {

.github/workflows/_run-example.yml

@@ -50,7 +50,7 @@ on:
 jobs:
   run:
     runs-on: ubuntu-24.04
-    container: ghcr.io/dfinity/icp-dev-env-${{ inputs.language }}:1.1.0
+    container: ghcr.io/dfinity/icp-dev-env-${{ inputs.language }}:v1.2.0
     env:
       ICP_CLI_GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
     steps:

AGENTS.md

@@ -491,9 +491,11 @@ Rust: `icp build backend && candid-extractor target/wasm32-unknown-unknown/relea
 ## Pending items (do not resolve prematurely)
 
 ### Container images
-Images are published at `ghcr.io/dfinity/icp-dev-env-{motoko,rust,all}`. Current pinned version: **`1.0.1`**. All devcontainer configs and CI workflows reference the pinned tag. When a new release is cut, update the tag in:
+Images are published at `ghcr.io/dfinity/icp-dev-env-{motoko,rust,all}`. Current pinned version: **`v1.2.0`**. Releases are tagged with a `v` prefix (e.g. `v1.2.0`); the registry publishes both `v1.2.0` and `1.2.0` tags, and we pin the `v`-prefixed form. When a new release is cut, update the tag in:
 - `.devcontainer/devcontainer.json`
-- `.github/workflows/*.yml`
+- `.github/workflows/_run-example.yml` (the single source of truth for the CI image version)
+
+`dev-env-version-check.yml` guards that these two stay in sync.
 
 Source: https://github.com/dfinity/icp-dev-env