Missing Package-Requires for agent-shell breaks isolated package builds

## Summary

`agent-shell-manager.el` directly requires `agent-shell`:

```elisp
(require 'agent-shell)
```

but the package does not currently declare `agent-shell` in package metadata.

This can break package managers/build systems that compile packages in isolated build environments, because `agent-shell` is not made available as a build/runtime dependency unless it is declared.

## Reproduction Context

I hit this through `nix-doom-emacs-unstraightened`, which builds Doom packages through isolated Nix derivations.

Adding `agent-shell-manager` caused the downstream package build to fail because `agent-shell-manager.el` requires `agent-shell`, but the generated package derivation did not know that `agent-shell` was required.

The relevant failure mode is:

```text
Cannot open load file: No such file or directory, agent-shell
```

or an equivalent package build failure while loading/byte-compiling `agent-shell-manager.el`.

## Expected Behavior

Package build systems should be able to infer that `agent-shell-manager` depends on `agent-shell` from standard Emacs package metadata.

## Proposed Fix

Add a `Package-Requires` header declaring the dependency:

```elisp
;; Package-Requires: ((emacs "25.1") (agent-shell "0"))
```

This preserves the README's stated Emacs baseline while making the existing `agent-shell` dependency explicit.

## Validation

In my downstream Nix/Doom configuration, injecting `agent-shell` as a build dependency for `agent-shell-manager` allowed the package derivation and full system build to complete successfully.

That suggests the missing dependency declaration is the cause of the isolated build failure.

Start in agent-shell-manager.el and inspect its package metadata alongside the existing require of agent-shell. Add the declared agent-shell dependency while preserving the stated Emacs baseline, then validate with an isolated package build or the downstream Nix/Doom configuration described in the issue.

Written by the indexing model from the issue text.