fix(sessions): use native Pi project directory encoding

[setkyar]

Summary

• write new sessions, forks, and clones to Pi's native per-project session directory
• keep reading directories written with pi-web's previous _-/__ escape encoding
• prefer the JSONL session header cwd when resolving lossy native directory names
• document the compatibility behavior and add regression coverage

Fixes #114.

Details

Pi encodes a project path by removing one leading separator, replacing /, \\, and : with -, and wrapping the result in --. pi-web used a lossless Unix-specific encoding instead, so Pi clients doing current-project discovery could miss sessions created by pi-web.

Native encoding is inherently ambiguous for literal hyphens. This change therefore treats the session header's cwd as authoritative and only decodes the directory name as a fallback. Existing pi-web directories remain readable.

Validation

• go test ./internal/sessions -count=1
• go test ./... -count=1
• go vet ./...
• gofmt clean
• git diff --check
• make build

make check reached the frontend suite but encountered unrelated 5-second timeout failures in SessionHeader, SessionSidebarProjects, and SessionTree; those three files passed independently (3 files, 11 tests). No frontend files are changed by this PR.