docs: add manual testing guide for slash command validation (#1955)

* docs: add manual testing guide for slash command validation

Adds a top-level TESTING.md that describes the manual test process PR
submitters must follow when their changes affect slash commands.

Includes:
- Process overview (identify affected commands, setup, run, report)
- Local setup instructions using editable install
- Reporting template for PR submissions
- Agent prompt that analyzes changed files and determines which
  commands need testing, including transitive script dependencies
  and extension hook mappings

* Update TESTING.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update TESTING.md

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

TESTING.md

@@ -0,0 +1,79 @@
+# Manual Testing Guide
+
+Any change that affects a slash command's behavior requires manually testing that command through an AI agent and submitting results with the PR.
+
+## Process
+
+1. **Identify affected commands** — use the [prompt below](#determining-which-tests-to-run) to have your agent analyze your changed files and determine which commands need testing.
+2. **Set up a test project** — scaffold from your local branch (see [Setup](#setup)).
+3. **Run each affected command** — invoke it in your agent, verify it completes successfully, and confirm it produces the expected output (files created, scripts executed, artifacts populated).
+4. **Run prerequisites first** — commands that depend on earlier commands (e.g., `/speckit.tasks` requires `/speckit.plan` which requires `/speckit.specify`) must be run in order.
+5. **Report results** — paste the [reporting template](#reporting-results) into your PR with pass/fail for each command tested.
+
+## Setup
+
+```bash
+# Install the CLI from your local branch
+cd <spec-kit-repo>
+uv venv .venv
+source .venv/bin/activate  # On Windows: .venv\Scripts\activate
+uv pip install -e .
+
+# Initialize a test project using your local changes
+specify init /tmp/speckit-test --ai <agent> --offline
+cd /tmp/speckit-test
+
+# Open in your agent
+```
+
+## Reporting results
+
+Paste this into your PR:
+
+~~~markdown
+## Manual test results
+
+**Agent**: [e.g., GitHub Copilot in VS Code]  |  **OS/Shell**: [e.g., macOS/zsh]
+
+| Command tested | Notes |
+|----------------|-------|
+| `/speckit.command` | |
+~~~
+
+## Determining which tests to run
+
+Copy this prompt into your agent. Include the agent's response (selected tests plus a brief explanation of the mapping) in your PR.
+
+~~~text
+Read TESTING.md, then run `git diff --name-only main` to get my changed files.
+For each changed file, determine which slash commands it affects by reading
+the command templates in templates/commands/ to understand what each command
+invokes. Use these mapping rules:
+
+- templates/commands/X.md → the command it defines
+- scripts/bash/Y.sh or scripts/powershell/Y.ps1 → every command that invokes that script (grep templates/commands/ for the script name). Also check transitive dependencies: if the changed script is sourced by other scripts (e.g., common.sh is sourced by create-new-feature.sh, check-prerequisites.sh, setup-plan.sh, update-agent-context.sh), then every command invoking those downstream scripts is also affected
+- templates/Z-template.md → every command that consumes that template during execution
+- src/specify_cli/*.py → CLI commands (`specify init`, `specify check`, `specify extension *`, `specify preset *`); test the affected CLI command and, for init/scaffolding changes, at minimum test /speckit.specify
+- extensions/X/commands/* → the extension command it defines
+- extensions/X/scripts/* → every extension command that invokes that script
+- extensions/X/extension.yml or config-template.yml → every command in that extension. Also check if the manifest defines hooks (look for `hooks:` entries like `before_specify`, `after_implement`, etc.) — if so, the core commands those hooks attach to are also affected
+- presets/*/* → test preset scaffolding via `specify init` with the preset
+- pyproject.toml → packaging/bundling; test `specify init` and verify bundled assets
+
+Include prerequisite tests (e.g., T5 requires T3 requires T1).
+
+Output in this format:
+
+### Test selection reasoning
+
+| Changed file | Affects | Test | Why |
+|---|---|---|---|
+| (path) | (command) | T# | (reason) |
+
+### Required tests
+
+Number each test sequentially (T1, T2, ...). List prerequisite tests first.
+
+- T1: /speckit.command — (reason)
+- T2: /speckit.command — (reason)
+~~~