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>

CLAUDE.md

@@ -66,7 +66,7 @@ Tests live alongside the code they test (`*_test.go`). The parser has both unit
 
 - **Parsing strategy**: User/system entries use byte-level extraction for speed; only assistant entries get full JSON parse (they carry token/cost data).
 - **Deduplication**: Messages are keyed by message ID; the final state wins (handles edits/retries).
-- **Cache**: SQLite at `~/.cache/cburn/sessions.db`. Mtime+size diffing means unchanged files aren't reparsed.
+- **Cache**: SQLite at `~/.cache/cburn/metrics_v2.db`. Mtime+size diffing means unchanged files aren't reparsed.
 - **TUI async loading**: Data loads via goroutines posting `tea.Msg`; the UI remains responsive during parse.
 - **Pricing**: Hardcoded in `internal/config/pricing.go` with user overrides in config TOML. Model names are normalized (date suffixes stripped).