revert: remove pre-Landlock bind mounts apply (#9300)

**Description**

This removes the pre‑Landlock read‑only bind‑mount step from the Linux
sandbox so filesystem restrictions rely solely on Landlock again.
`mounts.rs` is kept in place but left unused. The linux‑sandbox README
is updated to match the new behavior and manual test expectations.

codex-rs/linux-sandbox/README.md

@@ -6,52 +6,3 @@ This crate is responsible for producing:
 - a lib crate that exposes the business logic of the executable as `run_main()` so that
   - the `codex-exec` CLI can check if its arg0 is `codex-linux-sandbox` and, if so, execute as if it were `codex-linux-sandbox`
   - this should also be true of the `codex` multitool CLI
-
-## Git safety mounts (Linux)
-
-When the sandbox policy allows workspace writes, the Linux sandbox uses a user
-namespace plus a mount namespace to bind-mount sensitive subpaths read-only
-before applying Landlock rules. This keeps Git and Codex metadata immutable
-while still allowing writes to other workspace files, including worktree setups
-where `.git` is a pointer file.
-
-Protected subpaths under each writable root include:
-
-- `.git` (directory or pointer file)
-- the resolved `gitdir:` target when `.git` is a pointer file
-- `.codex` when present
-
-### How this plays with Landlock
-
-Mount permissions and Landlock intersect: if a bind mount is read-only, writes
-are denied even if Landlock would allow them. For that reason, the sandbox sets
-up the read-only mounts *before* calling `landlock_restrict_self()` and then
-applies Landlock rules on top.
-
-### Quick manual test
-
-Run the sandbox directly with a workspace-write policy (from a Git repository
-root):
-
-```bash
-codex-linux-sandbox \
-  --sandbox-policy-cwd "$PWD" \
-  --sandbox-policy '{"type":"workspace-write"}' \
-  -- bash -lc '
-set -euo pipefail
-
-echo "should fail" > .git/config && exit 1 || true
-echo "should fail" > .git/hooks/pre-commit && exit 1 || true
-echo "should fail" > .git/index.lock && exit 1 || true
-echo "should fail" > .codex/config.toml && exit 1 || true
-echo "ok" > sandbox-write-test.txt
-'
-```
-
-Expected behavior:
-
-- Writes to `.git/config` fail with `Read-only file system`.
-- Creating or modifying files under `.git/hooks/` fails.
-- Writing `.git/index.lock` fails (since `.git` is read-only).
-- Writes under `.codex/` fail when the directory exists.
-- Writing a normal repo file succeeds.