fix(workspace): Prevent session file collisions via source-prefixed keys

[JeanMertz]

Two sessions sharing a value but differing in source — e.g. $JP_SESSION=1234 alongside $TMUX_PANE=1234, or an Env value that matched a session-leader PID — resolved to the same 1234.json file, causing one session to silently overwrite the other's active conversation.

The filename now encodes both the session value and its source:

• getsid-<pid>.json
• hwnd-<handle>.json
• env-<KEY>-<hash(value)>.json

The opaque Env value is SHA-256 hashed (first 16 hex chars) so distinct variables holding the same value are kept apart and unsafe characters are excluded from the filename.

The session id is now stored inside the mapping itself, making each file self-describing. Stale detection decodes the id via typed SessionId::as_pid / SessionId::as_hwnd methods — inverses of the encoding in Session::getsid / Session::hwnd — instead of parsing the filename string. This closes the silent failure where a debug-formatted HWND (0x...) was stored but later parsed as decimal, yielding Unknown liveness and disabling stale detection.

Session::getsid and Session::hwnd now accept the raw typed value (i32 / isize) and own the encoding, keeping encode and decode in sync by construction. jp_cli passes the raw platform handle directly.

Existing bare-value files are handled without data loss: reads fall back to the legacy key when the source-prefixed key is absent, and stale-file cleanup migrates surviving legacy files to their new name in one pass.