docs: add "Adding a Framework Harness" guide and fix dangling workflow references - #426

What

getting_started/choose_your_agent_type.md and development_guides/streaming_patterns.md (both from #337) point readers to an agentex-add-agent-framework workflow that does not exist in this repo or in scale-agentex-python. This adds development_guides/adding_a_framework_harness.md: the files a new harness needs (tap + <Fw>Turn, exports, tests, agentex init templates and TemplateType registration, tutorials, docs page + nav), modelled on the Claude Code harness, and repoints both references. Adds the page to the nav under Framework Agents.

Test

mkdocs build --strict with docs/requirements.txt: the new page renders and links resolve; the 15 strict-mode warnings are pre-existing (a state_management.md link, two migration_guide.md anchors, griffe signature warnings) and none involve the changed files.

https://claude.ai/code/session_01HCVKnA7LeJZ44nxZz1uzF3

RetriggerConfidence Score: 5/5

The documentation-only PR appears safe to merge, with its new links, anchors, and navigation entry resolving correctly.

Summary

• Documents the tap, turn, tests, templates, tutorials, exports, and documentation expected for a new harness.
• Links the existing framework-selection and streaming guides to the new page.
• Adds the page under Development Guides → Framework Agents.

%%{init: {'theme': 'neutral'}}%%
flowchart LR
 A[Framework native event stream] --> B[Framework tap]
 B --> C[Canonical StreamTaskMessage events]
 C --> D[Framework HarnessTurn]
 D --> E[UnifiedEmitter or auto_send_turn]
 E --> F[Streaming delivery and derived spans]
 D --> G[Normalized usage]

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