Add fetch strategy support templates for HAF apps

Summary

Adds reusable templates for HAF apps that want to use GIT_STRATEGY=fetch instead of clone. Fetch strategy reduces GitLab server load by reusing workspace between jobs.

New templates

• .haf_fetch_strategy_variables - Variable definitions to copy (documentation)
• .haf_git_corruption_cleanup - Pre-get-sources hook for cleaning corrupt git state left by cancelled pipelines
• .haf_submodule_init - Manual HAF submodule initialization for nested hive submodules
• .haf_fetch_strategy_example - Shows all pieces together (documentation)

Usage

# In your .gitlab-ci.yml
include:
  - project: 'hive/common-ci-configuration'
    ref: develop
    file: '/templates/haf_app_testing.gitlab-ci.yml'

# Add corruption cleanup hook at default level
default:
  hooks: !reference [.haf_git_corruption_cleanup, hooks]

# In jobs that need submodules
my_job:
  before_script:
    - !reference [.haf_submodule_init, script]
    - # ... rest of before_script

Motivation

HAfAH switched to fetch strategy to reduce GitLab server load. This extracts that logic into reusable templates so other HAF apps can benefit.

Test plan

• HAfAH v3 branch uses these templates successfully
• Pipeline passes: https://gitlab.syncad.com/hive/HAfAH/-/pipelines/145910