MUL-6686 feat(daemon): use readable workdir paths (#7316)

Create new task environments under sanitized, bounded readable segments:
<workspace-slug>-<workspace-short>/<issue-key>-<task-short>/workdir.

The readable name is the visible part; the substance is moving task identity
off the directory name. A human-readable segment is mutable — workspace slugs
and issue prefixes change — so workspace/task identity now lives in
.task_owner, .gc_meta.json and a stable ID-keyed index under .task_roots that
freezes one physical root per task for its lifetime. GC and disk-usage read
that authoritative identity instead of inferring a workspace from a path
string, which also fixes active and preparation-crashed roots being attributed
to a nonexistent workspace.

Existing workdirs are not renamed or migrated, legacy roots are adopted into
the index, and both layouts render privacy-safe relative paths. Labels are
lowercased ASCII with separators and traversal collapsed, segments are capped
at 24 characters so the two together stay within the previous 49-character
Windows path budget, and stable ID suffixes drawn from the UUIDv7 random tail
keep roots distinct across renames and case-insensitive filesystems.

Closes #6063

Co-authored-by: iiwish <num2048@163.com>
Co-authored-by: J <bohan@devv.ai>