refactor(bindings): unify mount API, read-only by default

[chaliy]

Summary

• Safe default: mounts are read-only by default — callers must explicitly pass writable: true to enable host writes
• Unified API: JS and Python bindings now expose the same conceptual mount surface (files dict + mounts list + runtime mount()/unmount())
• Spec parity: added binding parity requirement to specs/003-vfs.md and maintenance checklist to specs/012-maintenance.md

Before → After

JS:

// Before: readOnly flag (unsafe if omitted)
mounts: [{ path: "/docs", root: "/real/path", readOnly: true }]
bash.mountReal(host, vfs, readOnly)

// After: writable flag (safe if omitted)
mounts: [{ path: "/docs", root: "/real/path" }]  // read-only
bash.mount(host, vfs)                              // read-only
bash.mount(host, vfs, true)                        // writable opt-in

Python:

# Before: 6 verbose kwargs
Bash(mount_real_readonly_at=[("/host", "/vfs")])

# After: unified mounts list
Bash(mounts=[{"host_path": "/host", "vfs_path": "/vfs"}])           # read-only
Bash(mounts=[{"host_path": "/host", "vfs_path": "/vfs", "writable": True}])

# Before: mount_text tuples
Bash(mount_text=[("/path", "content")])

# After: files dict
Bash(files={"/path": "content"})

Test plan

• All Rust mount tests pass (24 realfs + 7 live mount + 3 CLI)
• Python tests updated: files dict, mounts list, read-only default, writable, live mount/unmount
• JS tests updated: mount()/unmount() runtime methods
• New test_bash_mounts_readonly_by_default verifies safe default
• Type stubs (_bashkit.pyi) updated
• cargo check, cargo fmt, ruff check, ruff format all clean
• Python README updated
• Specs 003-vfs and 012-maintenance updated