Line slicing for curated context

[jamesrochabrun]

Parent: #398

Depends on: Context Builder MVP.

Goal

Allow curated context to include specific line ranges from large files instead of always including full file contents.

Implementation Notes

Add line-range support to the context model and assembler before adding rich editor interactions.

Suggested types/behavior:

• LineRange(start, end, description?), 1-based inclusive
• clamp ranges against current file content at assembly time
• merge overlapping or adjacent ranges
• assembled output labels file path and selected line ranges
• token estimates use sliced text, not full file contents

Editor integration can start small: if direct drag-selection in SourceCodeEditorView is too large for this issue, expose line slicing in the Context Builder preview first.

Acceptance Criteria

• A selected slice emits only that line range in the assembled context.
• Overlapping/adjacent slices merge deterministically.
• Stale ranges clamp safely when the file has changed.
• Full-file selection remains supported alongside sliced files.

Tests

Add pure tests for range normalization, merging, clamping, and assembled sliced output.