docs: add guide for handling complex features (#3004)

* docs: add guide for handling complex features

Add a Concepts page documenting strategies for dealing with large or
complex features where context window exhaustion degrades agent
performance during implementation. Covers limiting tasks per run,
sub-agent delegation, combining both, and decomposing into smaller
specs, with a guideline table for choosing an approach.

Closes #2986

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* docs: address review feedback on complex features guide

Use task IDs (T001-T010) instead of bare numbers to match the tasks.md
template format, and add the combined scoping + delegation approach to
the selection table for completeness.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

* docs: align complex features guide with command naming conventions

Use the full /speckit.implement command name throughout, match the
command template wording ('must consider'), and use the product names
GitHub Copilot CLI and the GitHub Copilot extension for VS Code.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Manfred Riem <mnriem@users.noreply.github.com>
Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

docs/toc.yml

@@ -43,6 +43,8 @@
       href: concepts/sdd.md
     - name: Spec Persistence Models
       href: concepts/spec-persistence.md
+    - name: Handling Complex Features
+      href: concepts/complex-features.md
 
 # Development workflows
 - name: Development