docs(huntsman): Add specification for task graph design.

[LinZhihao-723]

Description

This PR adds the specification for the task graph design, which defines the glossary needed for implementation.

The implementation requirements will be added in a future PR.

Checklist

• The PR satisfies the contribution guidelines.
• This is a breaking change and that has been indicated in the PR title, OR this isn't a
  breaking change.
• Necessary docs have been updated, OR no docs need to be updated.

Validation performed

• Ensure the doc can be rendered properly.
• Ensure all workflows pass.

Summary by CodeRabbit

• Documentation
  • Added a new "Task graph" section to developer docs describing tasks as a directed acyclic graph: task concepts, classifications, inputs/outputs, data‑flow and control‑flow dependencies, and execution scheduling rules.
  • Added a "Task graph" card/link on the developer docs index to surface the new section.

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[coderabbitai]

Walkthrough

Adds a new "Task graph" developer documentation page and registers it in the docs index with a grid card and toctree entry; documents Spider's task graph semantics (DAG, task metadata and classifications, data- and control-flow dependencies, and schedulability rules).

Changes

Cohort / File(s)	Summary
Developer documentation updates docs/huntsman/src/dev-docs/index.md, docs/huntsman/src/dev-docs/task-graph.md	Adds a "Task graph" grid item card to the dev-docs index and a new task-graph.md page; documents DAG-based task model, task metadata (TDL package, function name, retries), input/output/intermediate classifications, positional typed inputs/outputs, data-flow edges and fan-out, derived control-flow edges, and schedulability criteria.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The pull request title directly summarizes the main change: adding task graph design specification documentation.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.

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✨ Finishing touches

• 📝 Generate docstrings

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[coderabbitai]

Actionable comments posted: 1

🤖 Fix all issues with AI agents

In `@docs/huntsman/src/dev-docs/task-graph.md`:
- Around line 14-18: Fix the typo in the documentation bullet that currently
reads "* Other metadata such as maximum number of retires allowed." by changing
"retires" to "retries" so the line reads "* Other metadata such as maximum
number of retries allowed."; update the single occurrence of that phrase in the
task-graph documentation to correct the spelling.

🧹 Nitpick comments (1)

docs/huntsman/src/dev-docs/task-graph.md (1)

75-85: Use headings instead of emphasis for section titles.
MD036 flags these as headings; use proper heading levels for structure and consistency.

♻️ Proposed fix

-**Constraints on task inputs**
+##### Constraints on task inputs
...
-**Constraints on task outputs**
+##### Constraints on task outputs