fix: pre-authenticate the gitea-axi bench arm via its env interface

The gitea-axi arm was the only shell arm handed no credentials: the
runner set only PATH, so the agent had to reverse-engineer the tea-login
system — guessing a profile name and hunting for a config file — before
any real work, burning ~4 turns per task. Since turns drive cache-read,
the benchmark's dominant cost metric, this scaffolding gap alone inflated
gitea-axi's cost-equivalent tokens above every other arm.

Hand the arm its host and token through gitea-axi's own env interface
(GITEA_AXI_API_URL / GITEA_AXI_TOKEN), the symmetric counterpart to the
gitea-mcp server's GITEA_HOST / GITEA_ACCESS_TOKEN env: both name the
same two facts, and both still leave the agent to name the repository per
call. A shell arm now carries a credential env (empty for tea and
raw-api, which need none), merged under PATH in the driver.

Also strengthen SKILL.md so a cold agent targets and authenticates on the
first call: an explicit "Targeting and authentication" section replaces
the buried, optional-looking one-liner, spelling out that outside a
checkout `-R OWNER/NAME` plus the environment's token is all that is
needed — do not go hunting for a config file or login profile.

Verified live: create-memory-leak-issue dropped from 10 turns to 3 and
its cache-read fell ~3.8x, with the auth flailing gone from the transcript.

bench/sdk-driver.ts

@@ -271,7 +271,10 @@ function buildOptions(
   if (arm.shell !== null) {
     // Lead the agent's PATH with the arm's curated bin directory so only its one
     // allowed binary resolves by name; the guard on canUseTool is the authority.
-    options.env = { ...process.env, PATH: arm.shell.path };
+    // Layer the arm's credential env underneath so its tool is pre-authenticated
+    // the way its product is really configured, symmetric to the gitea-mcp
+    // server's env (see ArmShell.env); PATH stays last so it is never overridden.
+    options.env = { ...process.env, ...arm.shell.env, PATH: arm.shell.path };
   }
   if (arm.mcp !== null) {
     options.mcpServers = { [arm.arm]: { type: "stdio", ...arm.mcp.server } };