feat: replace fish integration files with built-in fish subcommand (task u6)

Add `foostore fish` subcommand that prints the complete fish shell integration
script (tab completions for foostore + the ge wrapper function) to stdout so
users can source it with `foostore fish | source`. Remove install-fish.sh,
FISH_INTEGRATION.md, and completions/ directory. Update CLAUDE.md and README.md
to document the new workflow.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

1 files changed, 4 insertions, 7 deletions

diff --git a/README.md b/README.md
index 501018a..2fa5de1 100644
--- a/README.md
+++ b/README.md
@@ -26,12 +26,11 @@ Tab completion and a `ge` shortcut wrapper are provided for the [fish shell](htt
 
 ### Install
 
-```bash
-./install-fish.sh
-exec fish
-```
+Source the integration script once to activate it for the current session, or add the line to `~/.config/fish/config.fish` for permanent activation:
 
-This copies `completions/foostore.fish` to `~/.config/fish/completions/` and `completions/ge.fish` to `~/.config/fish/functions/`.
+```fish
+foostore fish | source
+```
 
 ### Usage
 
@@ -58,8 +57,6 @@ ge <TAB>
 set -e PIN           # clear when done
 ```
 
-See `FISH_INTEGRATION.md` for more details.
-
 ## Interactive Picker UX
 
 In interactive shell mode (`foostore` with no arguments), pressing `Enter` on an empty line opens an enhanced fuzzy picker.