chore(security): externalise confidentiality term list

Moves the list of private organisation / pilot / codename
strings the security gate blocks OUT of the repo entirely.

Before: tools/security/check-staged.sh + the Today AI prompt
+ docs/today-pipeline.md held the strings in plaintext. The
whole point of the gate is to keep certain strings out of
committed artefacts, so holding them in a committed
artefact was self-defeating — anyone with read access to
the repo trivially recovered the very list we tried to
protect.

After:

- The gate reads a runtime file
  `${FAI_BANNED_TERMS_FILE:-~/.fai-security/banned-terms.txt}`
  at scan time. One regex per line, `#`-prefixed comments,
  matched case-insensitively against staged diffs and
  commit messages. Repos contain no copy.
- Pre-commit / commit-msg modes log a warning + skip the
  confidential-terms scan if the file is missing (fresh
  checkouts shouldn't trip until the operator bootstraps
  the list).
- CI mode (`check-staged.sh ci`) FAILS when the file is
  missing — runners are expected to be bootstrapped by
  their deploy step.
- The unit-test harness uses a synthetic placeholder term
  (`SYNTHETIC_BANNED_TERM_XYZZY`) injected via a temp
  banned-terms file, so the test never references real
  customer names.
- docs/today-pipeline.md + tools/today/prompt.template.md
  point at the runtime file instead of enumerating terms.

Operator bootstrap (one-time, per machine):

  mkdir -p ~/.fai-security && chmod 700 ~/.fai-security
  printf '\\b%s\\b\\n' TERM_A TERM_B > ~/.fai-security/banned-terms.txt
  chmod 600 ~/.fai-security/banned-terms.txt

Gate self-tests: 18 passed, 0 failed.

Signed-off-by: flemming-it <sf@flemming.it>
Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

tools/security/test-check-staged.sh

@@ -153,9 +153,19 @@ git add tests/server.pem
 assert_content_passes "tests/*.pem allowlisted"
 
 d=$(setup_repo); cd "$d"
-echo "Notes from the ITDZ Berlin meeting" > confidential.md
+# The confidentiality list lives outside the repo at
+# $FAI_BANNED_TERMS_FILE (default ~/.fai-security/banned-terms.txt).
+# For the test we point at a temp file holding a single
+# generic test term — the actual production terms stay in the
+# operator's untracked file and never enter this test suite.
+TEST_BANNED_TERMS=$(mktemp -t fai_banned.XXXXXX)
+printf "\\\\bSYNTHETIC_BANNED_TERM_XYZZY\\\\b\n" > "$TEST_BANNED_TERMS"
+export FAI_BANNED_TERMS_FILE="$TEST_BANNED_TERMS"
+echo "Document contains SYNTHETIC_BANNED_TERM_XYZZY in passing" > confidential.md
 git add confidential.md
-assert_content_fails "confidential term ITDZ" "ITDZ"
+assert_content_fails "confidential term (synthetic test term)" "SYNTHETIC_BANNED_TERM_XYZZY"
+rm -f "$TEST_BANNED_TERMS"
+unset FAI_BANNED_TERMS_FILE
 
 d=$(setup_repo); cd "$d"
 echo "Our viral release of a powerful tool that just works" > marketing.md