fix(cargo-agent): include file:line in diagnostics

[codesoda]

Summary

• Enable file and line location in check/clippy diagnostic output so agents can navigate directly to errors
• The extract_compiler_diags function already supported this via its include_location parameter — just needed to pass 1 instead of 0

Before:

error: this function has too many arguments (8/7)

After:

error: this function has too many arguments (8/7)
  --> src/lib.rs:42

Test plan

• All 4 cargo-agent scenarios pass (tests/run-scenarios.sh cargo-agent)
• Verified issues scenario log includes --> location lines

[cadence]

Cadence PR Review

Goal
Enable file:line location info in cargo-agent's check and clippy diagnostic output so agents can navigate directly to errors, by flipping an existing parameter from 0 to 1 in two call sites.

Key Decisions

• The extract_compiler_diags function already supported location output via its include_location parameter — no new parsing logic was needed, just toggling the flag.
• The awk '!seen[$0]++' deduplication works per-line, which remains correct since locations now produce an additional --> line per diagnostic.

Review Focus

Area	Why
skills/cargo-agent/scripts/cargo-agent.sh (lines 277, 318)	Both run_check and run_clippy changed from 0 to 1. Verify the extract_compiler_diags function's location format (especially the \n in jq output) produces clean multi-line output and that the per-line dedup via awk doesn't accidentally suppress location lines that are identical across different diagnostics.

Risk
Minimal risk — two single-character changes to an existing, tested code path. All four cargo-agent scenarios pass. Only concern is whether identical --> location lines for different errors could be deduped by awk, but this is unlikely in practice since the file:line will usually differ.

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View private review in Cadence