spec(worktree-subsystem): add specification for native worktree subsystem

[bug-ops]

Summary

• Produces the full spec package (BRD/SRS/NFR/spec/plan/tasks) for the native worktree management subsystem required by issue feat(subagent): worktree.baseRef config — branch from origin/<default> or local HEAD #4655
• Spec lives under specs/063-worktree-subsystem/, registered in specs/README.md
• No source code changed — spec-only PR

Design decisions encoded

C1 — cwd isolation (MVP: Constrain, don't isolate):

• INV-1: All worktree-attached runs serialized via a process-global CwdGuard mutex; non-worktree agents quiesced while held
• INV-2: Process cwd saved/restored (RAII) around agent run; restore happens BEFORE git worktree remove
• INV-3: set_working_directory in disallowed_tools for every worktree agent
• INV-4: Worktree creation failure aborts that agent's spawn — no silent fallback

Config schema:

[worktree]
enabled = false
base_ref = "head"          # "fresh" | "head"
default_branch = "main"
root = ".claude/worktrees"
branch_prefix = "agent/"
prune_branch_on_remove = false
cleanup_on_completion = true

Deferred:

• D1: True concurrent isolation via ToolExecutor cwd-threading
• D2: Child-process bgIsolation mode (tracked in feat(subagent): worktree.bgIsolation: none — allow background sessions to edit working copy directly #4656)

Architecture

New crate zeph-worktree (~930 LOC, 5 phases per plan.md). Integration with zeph-subagent, zeph-config, zeph-core.

Test plan

• All spec files validate (fy validate)
• BRD → SRS → spec → tasks traceability verified by reviewer
• INV-1..INV-4, M4, A1/A2, S2/S3, D1/D2 markers confirmed present in spec.md
• tasks.md acceptance criteria reviewed (16 tasks, concrete assertions)

Closes #4655