docs: improve installation guide with PATH setup

README changes:
- Add go install github.com/...@latest as primary install method
- Add git clone instructions as alternative
- Add PATH setup instructions for bash/zsh and fish shells
- Fix cache database filename (sessions.db -> metrics_v2.db)

CLAUDE.md:
- Fix cache database filename to match actual implementation

New users on fresh Go installations often miss that ~/go/bin needs
to be in PATH. This was causing "command not found" errors after
successful installation.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

README.md

@@ -5,15 +5,29 @@ A CLI and TUI dashboard for analyzing Claude Code usage metrics. Parses JSONL se
 ## Installation
 
 ```bash
-# Build from source
-make build
+go install github.com/theirongolddev/cburn@latest
+```
 
-# Install to ~/go/bin
+Or build from source:
+
+```bash
+git clone https://github.com/theirongolddev/cburn.git
+cd cburn
 make install
 ```
 
 Requires Go 1.24+.
 
+**Note:** Ensure `~/go/bin` is in your PATH:
+
+```bash
+# bash/zsh: add to ~/.bashrc or ~/.zshrc
+export PATH="$HOME/go/bin:$PATH"
+
+# fish: add to ~/.config/fish/config.fish
+fish_add_path ~/go/bin
+```
+
 ## Quick Start
 
 ```bash
@@ -145,7 +159,7 @@ To get your session key:
 
 ## Caching
 
-Session data is cached in SQLite at `~/.cache/cburn/sessions.db`. The cache uses mtime-based diffing - unchanged files are not reparsed.
+Session data is cached in SQLite at `~/.cache/cburn/metrics_v2.db`. The cache uses mtime-based diffing - unchanged files are not reparsed.
 
 Force a full reparse with `--no-cache`.