Add a two-module task manager example

task-manager.plain requires task-model.plain, demonstrating a requires
dependency chain: the base module is rendered first and the top module
builds on top of its generated code.

Author: NejcS

examples/task-manager/README.md

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+# Two-module task manager example
+
+This example is split into two modules to demonstrate a `requires` dependency chain:
+
+- **`task-model.plain`** — the base module. Defines `:Task:`, `:TaskList:`, and `:User:`,
+  implements the entry point, adding a task, and showing the task list. It exports its
+  concepts so downstream modules can build on them.
+- **`task-manager.plain`** — the top module. It `requires` `task-model`, inheriting its
+  generated code and functional specs as a starting point, then adds deleting, editing,
+  and completing tasks.
+
+Rendering `task-manager.plain` builds `task-model` first, then continues on top of it.
+
+# How to render the example
+
+You can run the example with the `codeplain` command:
+
+```bash
+codeplain task-manager.plain
+```
+
+# How to run the generated code
+
+After the rendering is finished, you can run the generated software code using the command:
+
+```bash
+python3 dist/taskmgr.py
+```
+
+See [top-level README](../../README.md) for additional information and help for troubleshooting the examples.

examples/task-manager/config.yaml

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+unittests-script: ../../test_scripts/run_unittests_python.sh
+conformance-tests-script: ../../test_scripts/run_conformance_tests_python.sh
+copy-build: true
+build-dest: dist

examples/task-manager/task-manager.plain

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+---
+description: 'A task manager application — extends the task model with editing features'
+import:
+  - python-console-app-template # spec source available at https://github.com/Codeplain-ai/codeplain/blob/main/standard_template_library/
+requires:
+  - task-model
+---
+
+***implementation reqs***
+
+- :MainExecutableFile: of :App: should be called "taskmgr.py".
+
+***test reqs***
+> Test requirements are defined in 'python-console-app-template'.
+
+***functional specs***
+
+- :User: should be able to delete :Task:
+
+- :User: should be able to edit :Task:
+
+- :User: should be able to mark :Task: as completed.
+
+
+
+> see https://www.plainlang.org/ for more information on ***plain

examples/task-manager/task-model.plain

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+---
+description: 'Core task model — base module providing tasks and a task list'
+import:
+  - python-console-app-template # spec source available at https://github.com/Codeplain-ai/codeplain/blob/main/standard_template_library/
+exported_concepts: [":User:", ":Task:", ":TaskList:"]
+---
+
+***definitions***
+
+- :User: is the user of :App:
+
+- :Task: describes an activity that needs to be done by :User:. :Task: has the following attributes
+  - Name - a short description of :Task:. This is a required attribute. The name must be at least 3 characters long.
+  - Notes - additional details about :Task:
+  - Due Date - optional date by which :User: is supposed to complete :Task:.
+
+- :TaskList: is a list of :Task: items.
+  - Initially :TaskList: should be empty.
+
+***implementation reqs***
+
+- :MainExecutableFile: of :App: should be called "taskmgr.py".
+
+***test reqs***
+> Test requirements are defined in 'python-console-app-template'.
+
+***functional specs***
+
+- Implement the entry point for :App:.
+
+- :User: should be able to add :Task:. Only valid :Task: items can be added.
+
+- Show :TaskList:
+
+
+
+> see https://www.plainlang.org/ for more information on ***plain