Add Learnings sections to instruction files

[Copilot]

Implements the learnings tracking system defined in learnings.instructions.md by adding standardized "## Learnings" sections to all instruction files.

Changes

• Added empty "## Learnings" sections to copilot-instructions.md, disposable.instructions.md, telemetry.instructions.md, and tree-widgets.instructions.md
• Converted existing learnings in observables.instructions.md from nested "Important learnings:" format to standardized "## Learnings" format with counters

All learnings now follow the format specified in learnings.instructions.md:

## Learnings

* Avoid glitches (1)
* Choose the right observable value type: Use `observableValue(owner, initialValue)` for regular values... (1)

This enables the learning system where counters track usefulness and learnings can be added when users request "learn!".

Original prompt

Implement the attached plan

The user has attached the following files as relevant context:

• .github/instructions/learnings.instructions.md
• .github/copilot-instructions.md

Created from VS Code via the GitHub Pull Request extension.

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