Identifiers in auto-tactics don't respect lexical scope

Prerequisites

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• Check that your issue is not already filed:
  https://github.com/leanprover/lean4/issues
• Reduce the issue to a minimal, self-contained, reproducible test case.
  Avoid dependencies to Mathlib or Batteries.
• Test your test case against the latest nightly release, for example on
  https://live.lean-lang.org/#project=lean-nightly
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Description

PR #3328 began allowing identifiers in auto-tactics, a much-desired feature. But these identifiers don't respect lexical scope/hygiene.

Steps to Reproduce

1. Create a file with the definition from the test case in #3328:

   def f3 (x y : Nat) (h : x = y := by exact Eq.refl x) : Nat :=
   x + x
   

2. Run the test with a local name other than x:

   #check fun someNameOtherThanX => f3 someNameOtherThanX someNameOtherThanX
   

Expected behavior: I would expect the test to pass, because x is bound by the function's parameter named x. Lean's tactic language usually respects lexical scope.

Actual behavior: Elaboration fails with the message unknown identifier 'x'

Versions

This is present in the 4.9 release, as well as 4.11.0-nightly-2024-07-01 on live.lean-lang.org.

Impact

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Start with the minimal reproducer from the issue and the definition and test case introduced by PR #3328. Trace how identifiers are handled in auto-tactics and verify that the local-name variant passes without an unknown-identifier error.