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.

docs/fish-config.md

@@ -1426,6 +1426,38 @@ commit them. Fisher installs and updates them automatically.
     jorgebucaran/fisher           Plugin manager itself
     meaningful-ooo/sponge         Remove failed commands from history
 
+## Sponge History Filtering
+
+Sponge removes failed commands from history and, via conf.d/sponge_privacy.fish,
+also filters privacy-sensitive commands through three layers:
+
+Layer 1 — Static patterns (universal, persistent across sessions):
+Commands matching any of these structural signatures are never recorded:
+
+    --password / --token / --passphrase / --api-key flags with values
+    Inline env assignments: GITHUB_TOKEN=xxx, MY_API_KEY=abc
+    Fish set with sensitive names: set -gx GITHUB_TOKEN xxx
+    URLs with embedded credentials: https://user:pass@host
+    HTTP Authorization headers: curl -H "Authorization: ..."
+    Basic auth flags: curl -u user:pass
+    sshpass, docker login -p, openssl -passin/-passout
+
+Layer 2 — Dynamic secret values (session globals, refreshed each login):
+On the first prompt, after secrets.fish has loaded, the literal values of
+all exported variables whose names suggest credentials (TOKEN, PASSWORD,
+SECRET, API_KEY, etc.) are collected, regex-escaped, and added as a
+session-scoped overlay.  Because globals shadow universals in Fish, the
+combined list is what sponge sees.  Rotating a token takes effect on the
+next login automatically.
+
+Layer 3 — Per-command filter (sponge_filter_secrets):
+Catches credentials in variables exported after login, such as tokens
+sourced from a project .env file mid-session.
+
+To add your own persistent patterns:
+
+    set -U -a sponge_regex_patterns 'your-regex-here'
+
 ## Bundled Plugin Functionality
 
 The remaining plugin functionality is bundled directly with this config rather