docs: rewrite soundpacks guide

[daiverd]

Summary

• Rewrites docs/soundpacks.md, verifying every claim against the actual source (internal/sounds/mapper.go, internal/hooks/parser.go, internal/soundpack/*.go, internal/cli/soundpack_*.go, internal/config/xdg.go) and, for the runtime-resolution behavior, against a locally built binary
• Fixes an inaccurate claim that hand-copied directory packs are auto-resolved at runtime — they aren't without a soundpack_paths entry (or a literal default_soundpack path); without one, Claudio silently falls back to the platform pack. New Discovery Vs. Runtime Resolution section documents this
• Adds a per-OS "Where Soundpacks Live" table (Linux/macOS/Windows/WSL) for directory packs, JSON packs, managed git clones, the git registry, and config.json
• Clarifies there's no shared soundpack marketplace — the "registry" is local bookkeeping for your own managed git packs
• Notes that soundpack update is pull-on-demand only, never automatic
• Reorders the whole page around reader intent instead of pack file format: Using A Pack (switch/install/remove, no authoring required) → Building A Pack (directory/JSON formats, validation, tracking) → How Sound Selection Works (fallback chains, command parsing — for debugging or extending Claudio)

Test plan

• Skim rendered page for broken anchors/links (all cross-references use kramdown auto-generated heading IDs, unaffected by heading-level changes)
• Spot-check the per-OS path table against internal/config/xdg.go / github.com/adrg/xdg

🤖 Generated with Claude Code