fix: preserve leading @ in tool paths

[skys-mission]

Problem

Tool paths beginning with a literal @ are resolved without that character. If both @scope/sample.txt and scope/sample.txt exist, read_file asked for the former silently reads the latter. write_file can likewise target the wrong directory.

Client reproduction

Create @scope/sample.txt containing EXPECTED and scope/sample.txt containing WRONG. Ask Step to call read_file with the exact path @scope/sample.txt. Before this fix, the tool returns 1: WRONG; afterward, it returns 1: EXPECTED.

Fix

Preserve the leading @ when resolving tool paths. CLI @file arguments are processed separately and retain their existing behavior.

Verification

• Added a Step tool profile regression test for reading and writing an @-prefixed path.
• Reproduced the issue and verified the fix in the Step client with the same read_file call.
• npm run check
• ./test.sh

Screenshots

Before fix: Step returns WRONG when asked to read @scope/sample.txt.

Reproduction files: @scope/sample.txt contains EXPECTED, while scope/sample.txt contains WRONG.

After fix: The same request returns EXPECTED.

[skys-mission]

Rechecked the documented @ path behavior; closing this PR because the proposed change would alter the intended behavior.