chore(op-devstack): fix rust binary error message and add debug build targets

[sebastianst]

Summary

• Fix misleading error message in rust_binary.go that referenced non-existent just build-rust-debug target — now dynamically suggests the correct target per binary (e.g. cd rust && just build-op-reth-debug)
• Add target/debug/ to the binary search path in resolveBuiltRustBinaryPath (release still takes priority)
• Add build-op-reth-debug and build-kona-node-debug just targets for faster local E2E test iteration
• Rename build-node to build-kona-node for consistency, keep build-node as alias

Test plan

• cd rust && just --list shows new targets and alias
• cd rust && just build-op-reth-debug builds successfully
• go vet ./op-devstack/sysgo/... passes
• E2E tests find debug binaries after just build-op-reth-debug

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.5%. Comparing base (63dce55) to head (3919172).
⚠️ Report is 3 commits behind head on develop.

❗ There is a different number of reports uploaded between BASE (63dce55) and HEAD (3919172). Click for more details.

HEAD has 10 uploads less than BASE

Flag	BASE (63dce55)	HEAD (3919172)
cannon-go-tests-64	1	0
contracts-bedrock-tests	9	0

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #19988      +/-   ##
===========================================
- Coverage     11.8%     0.5%   -11.4%     
===========================================
  Files          677      483     -194     
  Lines        71115    60263   -10852     
===========================================
- Hits          8440      320    -8120     
+ Misses       62531    59943    -2588     
+ Partials       144        0     -144     

Flag	Coverage Δ
cannon-go-tests-64	?
contracts-bedrock-tests	?
unit	0.5% <ø> (ø)

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see 194 files with indirect coverage changes

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