test: consent-gate E2E suite + functional fs-capability probes

Five hermetic gate-tier E2E cases (tpa-present / absent-linux / broken /
absent-darwin / apple-ban) drive the real contract section through
claude -p with PATH shims for aside and uname; the absent cases filter
any REAL aside binary out of the child PATH and assert absence with
Bun.which before spawning, so dev machines cannot leak into detection.
Registered per-case in E2E_TOUCHFILES/E2E_TIERS with template-level
deps (ship/SKILL.md.tmpl, gen-skill-docs.ts) and added to the evals.yml
matrix with tier: gate. eval:bg:periodic's detach timeout rises to
36000s for the grown periodic shard census (floor-enforced by
test/eval-detach-timeout-floor.test.ts); CLAUDE.md doc updated to match.

test/helpers/fs-caps.ts adds canRevokeWrites/canRevokeReads functional
probes; 13 chmod-based tests swap their uid-0-only guards for the
probes so suites skip honestly on CAP_DAC_OVERRIDE containers (this
sandbox: uid 1000 with full caps) instead of asserting revocations the
kernel ignores. path-validation's symlink test targets /etc/passwd
(exists everywhere; /etc/crontab is absent on Amazon Linux).

design/test/receipted-fetch.test.ts

@@ -11,6 +11,7 @@
  */
 
 import { describe, test, expect, beforeEach, afterEach } from "bun:test";
+import { canRevokeWrites } from "../../test/helpers/fs-caps";
 import fs from "fs";
 import os from "os";
 import path from "path";
@@ -108,7 +109,7 @@ describe("receiptedFetch", () => {
   });
 
   test("fail-open: unwritable ledger warns on stderr and the call proceeds", async () => {
-    if (process.platform === "win32" || process.getuid?.() === 0) return;
+    if (!canRevokeWrites()) return; // chmod is advisory here (win32, root, DAC-override containers)
     fs.mkdirSync(path.join(home, "security"), { recursive: true, mode: 0o500 });
     let fetched = false;
     const stub = (async () => { fetched = true; return new Response("{}", { status: 200 }); }) as typeof globalThis.fetch;