feat: add github-increase-copilot-user-budget script

[locus313]

Description

Adds a script to raise the Copilot AI credit budget for a single enterprise user. It shows the user's effective budget and consumption, offers +$50/+$100/+$150/+$200/+$250 presets, a custom increase, or an absolute amount, confirms, then PATCHes the user's AI credit budget or creates one when only the Universal budget applies.

Changes

• New script: enterprise/github-increase-copilot-user-budget/github-increase-copilot-user-budget.sh (plus action.yml)
• Modified: lib/github-common.sh
• Other: tests/test_script_validation.bats, README.md, new GLOSSARY.md

How to test

export GITHUB_TOKEN=ghp_REDACTED
export GITHUB_ENTERPRISE=test-enterprise
./enterprise/github-increase-copilot-user-budget/github-increase-copilot-user-budget.sh -u octocat --increase 100 --dry-run

Validated with bats tests/ (all passing, including 8 new tests) and shellcheck (clean). The create and PATCH flows were exercised against a mocked curl only; not yet run against a real enterprise.

Checklist

• set -euo pipefail is the first executable line after the # === header
• Script sources lib/github-common.sh via SCRIPT_DIR
• All required env vars validated with require_env_var
• Token validated with validate_github_token (or validate_token for secondary tokens)
• User-supplied slugs validated with validate_slug
• sleep added between repo-level operations to respect rate limits (n/a: single-user operation)
• README.md updated with the new/changed script (env var table + usage example)
• Script header comment matches README documentation
• Tested on a non-production org before production
• shellcheck passes (shellcheck --severity=warning --exclude=SC2034,SC1091 --shell=bash <script>)