feat(sponge): add three-layer privacy filtering for shell history

conf.d/sponge_privacy.fish registers patterns and filters that prevent
credentials from reaching fish history:

  Layer 1 (static regex, universal): auth flags, env assignments,
  credential-bearing URLs, Authorization headers, sshpass, docker
  login, openssl -passin/-passout

  Layer 2 (dynamic values, session globals): on the first prompt, after
  secrets.fish has loaded, reads the literal values of all exported
  credential-named vars (TOKEN, PASSWORD, SECRET, etc.), escapes them
  for regex, and merges them with the static patterns as a session
  global — auto-refreshes on login so rotated tokens are never stale

  Layer 3 (per-command filter, sponge_filter_secrets): catches
  credentials in variables exported mid-session (e.g. project .env)

Also exempts functions/sponge_filter_secrets.fish from the sponge_*
gitignore glob so our custom filter is committed alongside the config.

functions/sponge_filter_secrets.fish

@@ -0,0 +1,51 @@
+# Copyright (C) 2026 Rootiest
+# SPDX-License-Identifier: AGPL-3.0-or-later
+
+# SYNOPSIS
+#   sponge_filter_secrets <command> <exit_code> <previously_in_history>
+#
+# DESCRIPTION
+#   Custom sponge filter that prevents commands from being stored in history
+#   when they contain the literal value of any exported environment variable
+#   whose name indicates it holds a credential (TOKEN, PASSWORD, SECRET,
+#   API_KEY, etc.).  This catches shell-expansion leakage where a variable
+#   value is embedded directly in the command string at execution time — a
+#   case that static regex patterns cannot cover.
+#
+#   Any variable whose name matches the sensitive-name heuristic and whose
+#   value is longer than 8 characters (excluding bare paths) is checked.
+#   The value is escaped for literal regex matching before comparison.
+#
+# ARGUMENTS
+#   command                 The exact command that was entered
+#   exit_code               Exit code of the command (unused)
+#   previously_in_history   "true"/"false" flag (unused)
+#
+# RETURNS
+#   0  Command contains a secret value — filter out of history
+#   1  No secret value found — keep in history
+#
+# EXAMPLE
+#   # Register with sponge (done automatically by conf.d/sponge_privacy.fish):
+#   set -U -a sponge_filters sponge_filter_secrets
+
+function sponge_filter_secrets --argument-names command
+    # Find all exported variables with security-sensitive names
+    set -l sensitive_vars (set --names --export | string match --regex -- \
+        '(?i)(?:TOKEN|PASSWORD|PASSWD|SECRET|API[_-]KEY|PRIVATE[_-]KEY|ACCESS[_-]KEY|AUTH[_-]KEY|CREDENTIAL|KOPIA_PASSWORD)')
+
+    for var in $sensitive_vars
+        set -l value $$var
+
+        # Skip empty, short, or path-like values — not real credentials
+        test (string length -- $value) -gt 8; or continue
+        string match --quiet --regex '^[/~]' -- $value; and continue
+
+        # Filter if the literal value appears anywhere in the command
+        if string match --quiet --regex -- (string escape --style=regex -- $value) $command
+            return 0
+        end
+    end
+
+    return 1
+end