Updates commit message instructions in auto-compose

[axosoft-ramint]

The commit message section of the auto-compose prompt now more closely matches the existing detailed instructions we had in the generate commit message action.

Note that this cannot be an exact copy-paste/injection of the generate-commitMessage prompt itself into the generate-commits prompt because there are some subtle differences:

• The message is output as a single string rather than a separated summary and explanation section
• The "explanation" part had to be renamed to "details" so the AI agent does not confuse it with the separate "explanation" it needs to output for each commit to the composer.

Closes #4870

[augmentcode]

🤖 Augment PR Summary

Summary: This PR refines the Commit Composer auto-compose prompt so its commit-message guidance aligns more closely with the existing “generate commit message” action.

Changes:

• Expands the generate-commits prompt with more explicit, step-by-step commit message instructions (subject line + blank line + details + optional issue reference).
• Updates generateCommits to incorporate both commit-message and commit-organization custom-instruction settings with clear priority rules, plus optional per-session guidance.
• Adds telemetry fields for the commit-message custom-instructions setting (used/length) and wires them into composer webview events.
• Updates telemetry type definitions and documentation to reflect the new event properties.

Technical Notes: The composed prompt still outputs a single commit message string, while keeping the per-commit explanation field separate for reviewer-oriented detail.

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[augmentcode]

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[axosoft-ramint]

Note that the user can provide custom instructions for commit messages in three places now:

1. Generate commit message setting
2. Generate commits setting
3. Input box when clicking auto-compose

Updated the instructions to prioritize them in order from lowest to highest in this order.

[Copilot]

Pull request overview

This PR updates the commit message generation instructions in the auto-compose feature to align more closely with the existing detailed instructions from the generate commit message action. The changes enhance the AI's ability to produce high-quality commit messages by providing more comprehensive guidance on structure, tone, and content.

Changes:

• Expands commit message generation guidelines in the prompts with detailed steps covering analysis, composition rules, formatting, and issue references
• Integrates commit message custom instructions into the generateCommits action with proper priority handling
• Adds telemetry tracking for commit message custom instructions usage and length

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/plus/ai/prompts.ts	Expands commit message generation guidelines with detailed multi-step instructions replacing the previous brief guideline
src/plus/ai/actions/generateCommits.ts	Refactors custom instructions handling to incorporate commit message settings with priority ordering
src/webviews/plus/composer/composerWebview.ts	Adds telemetry event data fields for tracking commit message custom instructions
src/constants.telemetry.ts	Defines new telemetry event properties for commit message custom instructions
docs/telemetry-events.md	Documents the new telemetry fields across multiple event types

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