Centralize agent instructions in SPEC (#4912)

* Centralize agent instructions in SPEC

Move the full coding-agent contract and repository guidance from `AGENTS.md` into `SPEC.md`, and reduce `AGENTS.md` to a simple pointer to the canonical instructions. Add lightweight `CLAUDE.md`, `GEMINI.md`, and `.github/copilot-instructions.md` files so different assistants consistently bootstrap through the same repo-specific guidance.

* Clarify auto-generated docs path scope in SPEC

Narrowed rule #3 to specify exact auto-generated subdirectories (tweaks/ and features/) rather than the entire code-reference/ directory, and clarified that other hand-written pages (e.g. architecture.mdx) may be edited directly.

* Remove archived docs from gitignore

Drop the old `docs-old` Hugo ignore rules from `.gitignore`, reflecting that the archived docs build artifacts no longer need special handling there.

* Add local env files to docs .gitignore

Add .env.local and .env.*.local patterns to prevent local environment files from being committed.

* Clarify test and git hygiene guidance

Update `SPEC.md` to run Pester in CI mode and tighten repository hygiene instructions around ignored files. The git guidance now points contributors to the actual `.gitignore` files and clarifies that `docs/public/` contains tracked static assets rather than generated output.

* Split agent workflow from project spec

Move repository working instructions into `AGENTS.md` and refocus `SPEC.md` on the stable WinUtil project contract. This separates agent-specific guidance from architecture, build, runtime, docs, testing, and release details so both documents have clearer ownership.

* docs: clarify Pester, ScriptAnalyzer, and source-of-truth rules

Expand AGENTS.md with context on why -SkipPublisherCheck is needed for Pester installation, why winutil.ps1 should be deleted before running ScriptAnalyzer, and clarify that the source-of-truth rule applies only to compiled-script behavior — repository metadata files are edited directly.

* Clarify Pester install command and -SkipPublisherCheck reason

Expands the explanation for why -SkipPublisherCheck is needed (catalog-signed vs Authenticode-signed), clarifies it does not skip download integrity, and adds -Repository PSGallery to pin the trusted source explicitly.

GEMINI.md

@@ -0,0 +1,3 @@
+# GEMINI.md
+
+Read `AGENTS.md` in the repository root for operating instructions before doing anything else.