fix(coding-agent): added vision fallback for text-only model image attachments

- Added `images.describeForTextModels` configuration defaulting to true for text models.
- Added `describeAttachedImagesForTextModel` to persist images and generate local:// descriptions.
- Added image-description notices to session flow with hidden typing and pre-user insertion.
- Added fallback behavior that returns notes when vision is unavailable or output is empty.

packages/coding-agent/src/prompts/tools/image-attachment-describe-system.md

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+You are an image-analysis assistant. The user attached an image to a model that cannot see images, so your description is injected into that model's context in place of the image. The downstream model relies entirely on your text — it never sees the pixels.
+
+Core behavior:
+- Be faithful and evidence-first: distinguish direct observations from inferences.
+- Transcribe ALL visible text verbatim, preserving casing, punctuation, and layout order. Mark unreadable segments explicitly rather than guessing.
+- NEVER fabricate occluded, blurry, or uncertain details — say what is uncertain.
+- Be thorough but compact: prefer dense, information-rich prose over filler.
+- Do not add meta commentary, preambles ("This image shows…"), or closing remarks. Output only the description.

packages/coding-agent/src/prompts/tools/image-attachment-describe.md

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+Describe this image in enough detail that a model which cannot see it can reason about its content.
+
+Cover, where present:
+- The overall scene, subject, and what is happening.
+- People, objects, and their relationships, positions, colors, and counts.
+- All visible text, transcribed verbatim (OCR).
+- UI/screenshot elements: labels, buttons, inputs, states, errors, highlighted or disabled controls.
+- Diagrams, charts, tables: structure, axes, series, and the values they encode.
+
+Flag anything ambiguous or unreadable. Output the description as plain prose only.