fix: reopen writable linux carveouts under denied parents

[viyatb-oai]

Summary

• preserve Linux bubblewrap semantics for write -> none -> write filesystem policies by recreating masked mount targets before rebinding narrower writable descendants
• add a Linux runtime regression for /repo = write, /repo/a = none, /repo/a/b = write so the nested writable child is exercised under bubblewrap
• document the supported legacy Landlock fallback and the split-policy bubblewrap behavior for overlapping carveouts

Example

Given a split filesystem policy like:

"/repo" = "write"
"/repo/a" = "none"
"/repo/a/b" = "write"

this PR keeps /repo writable, masks /repo/a, and still reopens /repo/a/b as writable again under bubblewrap.

Testing

• just fmt
• cargo test -p codex-linux-sandbox
• cargo clippy -p codex-linux-sandbox --tests -- -D warnings