Type in-container paths as POSIX on every host

[lexoliu]

In-container paths were built with host PathBufs, so on a Windows host the tool mount became /opt/aither/tools\<key>\bin inside a Linux container.

• New public ContainerPath (wrapping typed-path 0.12's POSIX path): absolute only, always joined with /. Used for the tool mount directory and its bin, the container side of MountSpec/MountRoot, the missing read-only mount error and the container mount check — a host path no longer type-checks there.
• Prepared tools are typed by where they run (host path for local/SSH, ContainerPath for containers); terminal.rs passes one value per backend instead of five optional fields, removing three runtime "missing runtime/target" errors.
• Tests without a platform gate assert /opt/aither/tools/<key> and /opt/aither/tools/<key>/bin exactly, POSIX joins of a Windows-style relative segment, rejection of relative paths and JSON round-trip.

Downstream (May): profile mount container paths parse with ContainerPath::new (relative paths are rejected), and one .display() becomes .to_string().

fmt, workspace clippy -D warnings and aither-sandbox (175 tests) pass on macOS; Windows is covered by the portability CI job.

Closes #135

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 76.84564% with 69 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
sandbox/src/terminal.rs	34.66%	49 Missing ⚠️
sandbox/src/tool_environment.rs	75.55%	11 Missing ⚠️
sandbox/src/tool_runtime.rs	91.54%	6 Missing ⚠️
sandbox/src/container.rs	96.77%	3 Missing ⚠️

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