feat(docs): add auto-generated Markdown wiki

Adds docs/split-wiki.py, a Python script that splits fish-config.md
into a multi-page Markdown wiki under docs/wiki/:

- index.md: DESCRIPTION intro + full section ToC
- one file per numbered section (1-configuration-variables.md … 9-viewing-this-manual.md)
- each section page has a nav bar at the top linking to every other section

The build-docs.yml workflow now runs the script alongside pandoc so all
three formats (man page, HTML, wiki) are regenerated together from a
single source in one CI run.

README slimmed down to a pointer to the wiki; docs/fish-config.md §9
gains an "As a wiki" subsection.

docs/wiki/1-configuration-variables.md

@@ -0,0 +1,51 @@
+# 1. CONFIGURATION VARIABLES
+
+**Sections:** [Index](index.md) | **1. Configuration Variables** | [2. Path Setup](2-path-setup.md) | [3. Key Bindings](3-key-bindings.md) | [4. Abbreviations](4-abbreviations.md) | [5. Functions Reference](5-functions-reference.md) | [6. Dependency Catalog](6-dependency-catalog.md) | [7. Customization](7-customization.md) | [8. Fisher Plugins](8-fisher-plugins.md) | [9. Viewing This Manual](9-viewing-this-manual.md)
+
+---
+
+These variables are exported from config.fish on every interactive session.
+Override them in ~/.config/.user-dots/fish/local.fish.
+
+## Environment Directories (XDG)
+
+    XDG_CONFIG_HOME    ~/.config
+    XDG_CACHE_HOME     ~/.cache
+    XDG_DATA_HOME      ~/.local/share
+    XDG_STATE_HOME     ~/.local/state
+
+Tools that respect XDG are directed to these paths rather than polluting $HOME.
+
+## Tool Homes (XDG-compliant)
+
+    CARGO_HOME         $XDG_DATA_HOME/cargo
+    RUSTUP_HOME        $XDG_DATA_HOME/rustup
+    GOPATH             $XDG_DATA_HOME/go
+    BUN_INSTALL        $XDG_DATA_HOME/bun
+    NPM_CONFIG_PREFIX  $XDG_DATA_HOME/npm-global
+    GNUPGHOME          $XDG_CONFIG_HOME/gnupg
+    WAKATIME_HOME      $XDG_CONFIG_HOME/wakatime
+
+## Editor and Pager
+
+    EDITOR      nvim (falls back to vi if nvim is absent)
+    VISUAL      same as EDITOR
+    SUDO_EDITOR same as EDITOR
+    PAGER       ov (falls back to less)
+
+## Scrollback History
+
+    SCROLLBACK_HISTORY_DIR        ~/.terminal_history
+    SCROLLBACK_HISTORY_MAX_FILES  100
+
+Scrollback logs accumulate in SCROLLBACK_HISTORY_DIR as timestamped files.
+When the count exceeds SCROLLBACK_HISTORY_MAX_FILES the oldest are pruned
+automatically on exit. Use `logs` to browse them interactively.
+
+## Other
+
+    GPG_TTY              $(tty)  — ensures GPG passphrase prompts work
+    CLAUDE_CODE_NO_FLICKER  1    — suppress terminal flicker in Claude Code
+    CDPATH               . ~/projects ~
+
+---