feat: gate set-env/add-path and render annotation locations (#1109)

`::set-env::` and `::add-path::` let a step rewrite the environment of every later step from its own output, which the runner honoured silently. They are now refused, as GitHub has done since 2020, and `ACTIONS_ALLOW_UNSECURE_COMMANDS` opts back in per step or job. Support for that variable is new here too, and is the only opt-in, matching GitHub rather than adding a runner config key on top.

Annotations keep their source location: Gitea has no annotation store and its web UI strips command properties, so `::error file=main.go,line=12::msg` is rendered as `::error::main.go:12: msg`.

`DEVELOPMENT.md` writes down the log line encoding rules this relies on.

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Co-authored-by: silverwind <me@silverwind.io>
Reviewed-on: https://gitea.com/gitea/runner/pulls/1109
Reviewed-by: silverwind <2021+silverwind@noreply.gitea.com>
Co-authored-by: bircni <bircni@icloud.com>

AGENTS.md

@@ -8,6 +8,7 @@
 - Never force-push, amend, or squash unless asked. Use new commits and normal push for pull request updates
 - Comments: write almost none, short and preferably same-line, explaining why for a future reader. Never narrate code, the change or the prompt. Preserve existing ones that still apply
 - Add the current year into the copyright header of new `.go` files
+- Read `DEVELOPMENT.md` for internals and conventions
 - Ensure no trailing whitespace in edited files
 - Run `make fmt` after `.go` edits, `make tidy` after `go.mod` edits, and `make checks` for the non-lint source checks
 - Lint what changed with `make lint-go`, and `make lint-go-windows` for Windows and platform-split files