chore: redact account emails from sample-run docs + ignore filter-repo backup

Replaces three real farm-domain emails in tracked markdown with
example.com (canonical IETF reserved domain) so the docs no longer
identify the OAuth provider behind the test pool.

- tools/onboard/README.md: alice/bob/<redacted>@example.com -> @example.com
  (the names were already pseudonyms; only the domain was real-pool-leaking)
- .sisyphus/reports/phase-g-performance-postmortem.md: '<redacted>@example.com'
  -> 'account_id elided' (full identifier was a real pool account)
- .gitignore: ignore /.git.backup-pre-filter-repo-*/ so a stray git add
  never re-commits the 169MB filter-repo safety backup

This is the source-side half of the email scrub. The history-side
(rewriting old commits where these strings originally landed) runs as
a separate filter-repo --blob-callback pass after this commit.

Author: nopperabbo

.gitignore

@@ -48,3 +48,6 @@ research-v3/enowx-reference/
 /.sisyphus/notes/*.log
 /internal/server/mansion/client/build.log
 
+
+# filter-repo backup directories (created by user during pre-release scrub; never commit)
+/.git.backup-pre-filter-repo-*/

.sisyphus/reports/phase-g-performance-postmortem.md

@@ -52,7 +52,7 @@ Ditto: **identical per-domain averages**. No Google-side rate-limit gradient det
 
 ## Where the 237 seconds go
 
-Sample run (`<redacted>@example.com`, 187s end-to-end):
+Sample run (account_id elided, 187s end-to-end):
 
 | Phase | Start | End | Duration | % | Source |
 |---|---|---|---|---|---|

tools/onboard/README.md

@@ -208,15 +208,15 @@ pending and retried.
 
 ```
 starting batch: 76 accounts, state=batch_state.json, output=kiro_tokens.json
-[1/76] DONE <redacted>@example.com (42.1s, attempt=1)
-[2/76] TRANSIENT <redacted>@example.com (browser, attempt=1/3) — will retry
-[3/76] FAILED <redacted>@example.com (blocked, attempt=1)
+[1/76] DONE alice@example.com (42.1s, attempt=1)
+[2/76] TRANSIENT bob@example.com (browser, attempt=1/3) — will retry
+[3/76] FAILED carol@example.com (blocked, attempt=1)
 ...
 ────────────────────────────────────────────────────────────
 Batch complete: 69/76 succeeded (failed=7, skipped=0)
 
 Failed:
-  <redacted>@example.com — hard:blocked (attempts=1)
+  carol@example.com — hard:blocked (attempts=1)
   ...
 
 Resume: re-run the same command. Transient failures will