Move the agent guidance from CLAUDE.md to AGENTS.md

REPO_POLICIES.md keeps guidance for coding agents in one root AGENTS.md,
never in a file named for one agent tool. The content moves unchanged,
and the TODO.md entry says so.

Model: opus-5-5

TODO.md

@@ -30,6 +30,26 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-15 the canonical files are re-vendored from `sneak/prompts` commit
+  `e3f79c4` (closes #221): `.dockerignore`, `.editorconfig`,
+  `.gitea/workflows/check.yml`, `.gitignore`, `.golangci.yml`,
+  `.prettierignore`, `.prettierrc`, `REPO_POLICIES.md` and the standard scripts
+  are its copies. pixa's own entries follow the standard part: `/bin`, `/data`
+  and the local config files in `.dockerignore`; Go's `*.log`, `*.out` and
+  `*.test`, `vendor/`, the binaries, `/data/`, `*.sqlite3` and the local config
+  files in `.gitignore`. The workflow keeps its `script/docker-smoke` step while
+  https://git.eeqj.de/sneak/pixa/issues/203 is open. The scripts pass a new
+  `CACHEBUST` build argument in place of `--no-cache`, and every `Dockerfile`
+  stage declares it first, so every `RUN` step still runs on every build while
+  Go's build cache, mounted on the steps that compile, is kept; both `go test`
+  runs keep `-count=1`. The lint and test builds write no image. The lint phase
+  runs golangci-lint v2.14.0, the test phase runs on the Debian Go image, and
+  the build stage marks `/src` safe for git. `script/bootstrap`, `script/fmt`
+  and `script/fmt-check` use the node on the `PATH` only when it has the pinned
+  major version. Of golangci-lint v2.14.0's two findings, an unused `nolint`
+  directive is removed, and gosec's new path check is marked as not applying to
+  the fixed config file locations. The agent guidance moves unchanged from
+  `CLAUDE.md` to a root `AGENTS.md`, where `REPO_POLICIES.md` keeps it.
 - 2026-10-15 the request metrics record any method that is not a standard one as
   `other` (closes #274): their `method` label was each request's method as sent,
   and Go's HTTP server accepts any method name, so a client sending made-up
@@ -56,25 +76,6 @@ P2: security: per-origin rate limiting
   the cookie, as anyone can take a CSRF token from the login page. A logout that
   cannot be saved answers 500. `session.NewManager`, which the tests use, keeps
   the value in memory only; pixad uses `session.NewManagerWithDatabase`.
-- 2026-10-15 the canonical files are re-vendored from `sneak/prompts` commit
-  `e3f79c4` (closes #221): `.dockerignore`, `.editorconfig`,
-  `.gitea/workflows/check.yml`, `.gitignore`, `.golangci.yml`,
-  `.prettierignore`, `.prettierrc`, `REPO_POLICIES.md` and the standard scripts
-  are its copies. pixa's own entries follow the standard part: `/bin`, `/data`
-  and the local config files in `.dockerignore`; Go's `*.log`, `*.out` and
-  `*.test`, `vendor/`, the binaries, `/data/`, `*.sqlite3` and the local config
-  files in `.gitignore`. The workflow keeps its `script/docker-smoke` step while
-  https://git.eeqj.de/sneak/pixa/issues/203 is open. The scripts pass a new
-  `CACHEBUST` build argument in place of `--no-cache`, and every `Dockerfile`
-  stage declares it first, so every `RUN` step still runs on every build while
-  Go's build cache, mounted on the steps that compile, is kept; both `go test`
-  runs keep `-count=1`. The lint and test builds write no image. The lint phase
-  runs golangci-lint v2.14.0, the test phase runs on the Debian Go image, and
-  the build stage marks `/src` safe for git. `script/bootstrap`, `script/fmt`
-  and `script/fmt-check` use the node on the `PATH` only when it has the pinned
-  major version. Of golangci-lint v2.14.0's two findings, an unused `nolint`
-  directive is removed, and gosec's new path check is marked as not applying to
-  the fixed config file locations.
 - 2026-10-15 a cached file left empty or partial is no longer served, or fetched
   again on every request (closes #252). Source and transformed image files, and
   each transformed image's `.meta` file, are synced to disk before they are