Add examples for using use-witness with environment variables as emergency escape valve

[Copilot]

Provides documentation and examples for the use-witness parameter to quickly disable witness attestation during service outages (e.g., Fulcio down) without requiring a pull request.

Changes

Examples added (examples/ directory):

• use-witness-with-env-var.yml - Basic repository variable pattern
• emergency-disable-witness.yml - Multi-job emergency disable pattern
• advanced-witness-control.yml - Fork-aware, branch-specific, and combined logic patterns
• README.md - Setup instructions, use cases, best practices, and troubleshooting

Main README updated:

• Added "Conditional Attestation with use-witness" section
• Emergency disable procedure (set repository variable → immediate effect)
• Link to examples

Usage

Control witness attestation via repository variable (Settings → Secrets and variables → Actions → Variables):

jobs:
  build:
    uses: testifysec/witness-run-action/.github/workflows/witness.yml@main
    with:
      # Disable by setting WITNESS_ENABLED='false' in repository settings
      use-witness: ${{ vars.WITNESS_ENABLED != 'false' }}
      step: build
      attestations: "git github environment"
      command: make build

Defaults to enabled when variable is unset. Change variable value to disable globally across all workflows without code changes.

Original prompt

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This section details on the original issue you should resolve

<issue_title>chore: provide examples for users to use use-witness</issue_title>
<issue_description>provide examples for users to use use-witness with a env var configured for the action.

this is an escape valve for non-transient errors that might occur when using witness-run-action

for instance, if the public fulcio goes down, or something else downstream, this can be used to quickly disable witness without a pull request to unblock pipelines. </issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes chore: provide examples for users to use use-witness #44

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