fix(bootstrap): handle tar paths exceeding 100 bytes

[johntmyers]

Summary

• Replace manual Header::set_path() + append() with builder.append_path_with_name() which emits GNU LongName extensions for paths exceeding the 100-byte POSIX tar name field limit
• Remove ~30 lines of manual header/read/append logic in favor of a single API call already used in crates/openshell-cli/src/ssh.rs
• Add regression test with a path exceeding 100 bytes

Fixes #705

Test plan

• New test_long_path_exceeding_100_bytes test passes — creates a deeply nested path >100 bytes and verifies it appears in the tar archive
• All 7 existing build::tests pass with no changes
• Manual smoke test: openshell sandbox create --from with a build context containing deeply nested node_modules/ paths