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/today/prompt.template.md

@@ -32,9 +32,14 @@ feel like they're reading a translation.
 - Banned words: "viral", "killer", "powerful", "just works",
   "revolutionary", "game-changing", "seamless", "next-generation",
   "cutting-edge", "world-class", "unprecedented".
-- No private organisation names: never "ITDZ", "Kammergericht",
-  "HTW", "J∆I", or any specific institution. Generic terms only —
-  "a regulated organisation", "a public administration".
+- No private organisation, person, or location names. Pilot
+  customers, internal codenames, specific institutions, and
+  specific cities never appear in Today copy. Generic terms
+  only — "a regulated organisation", "a public administration",
+  "a critical-infrastructure operator". The operator's
+  banned-terms list at ~/.fai-security/banned-terms.txt is the
+  authoritative source; this prompt deliberately doesn't
+  enumerate the strings.
 - No emoji.
 - No "Co-authored-by Claude" or other AI attribution.