ci: pin lock-threads to v6.0.2 to fix token validation failure

[simpleqt]

Problem

The scheduled Lock Threads workflow fails on every run — most recently run 35936703327 (2026-09-24), and on each preceding scheduled execution.

The failure comes from inside the action:

##[error]"github-token" length must be less than or equal to 100 characters long

The pinned revision (v5-era commit 1bf7ec2) still carries the upstream github-token validation schema that caps the input at 100 characters. The automatically-injected GITHUB_TOKEN now exceeds that length, so the action rejects its default input before locking anything — closed threads have stopped being locked.

Upstream fixed the validation schema in v6.0.2 ("update github-token validation schema", closes dessant/lock-threads#55).

Fix

Re-pin the action to the v6.0.2 release commit (89ae32b0). None of the inputs used in this workflow changed between the pinned revision and v6.0.2, so lock behaviour stays identical once the workflow can run again.

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