feat(agentbuilder): harden generated skill prompt boundaries

[Alan-TheGentleman]

🔗 Linked Issue

Closes #576

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🏷️ PR Type

What kind of change does this PR introduce?

• type:bug — Bug fix (non-breaking change that fixes an issue)
• type:feature — New feature (non-breaking change that adds functionality)
• type:docs — Documentation only
• type:refactor — Code refactoring (no functional changes)
• type:chore — Build, CI, or tooling changes
• type:breaking-change — Breaking change (fix or feature that changes existing behavior)

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📝 Summary

This PR pilots XML-like prompt boundaries in the custom agent builder. It wraps volatile prompt payloads in explicit tags while preserving the generated artifact contract: the model must still output raw SKILL.md Markdown.

What this gains:

• clearer separation between system-level generation instructions and dynamic payloads,
• less accidental blending between user requests, SDD context, and authoritative prompt instructions,
• testable prompt boundaries for future prompt-composition changes,
• delimiter-breakout hardening for values that contain closing tags like </user_request>.

This does not convert skills, docs, installer assets, or SDD assets away from Markdown.

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📂 Changes

File / Area	What Changed
internal/agentbuilder/prompt.go	Wraps installed-agent context, SDD context, and user request in XML-like tags. Escapes dynamic values with html.EscapeString.
internal/agentbuilder/prompt_test.go	Updates prompt-composition tests for wrapper presence/absence, raw SKILL.md output preservation, and delimiter-breakout cases.

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🧪 Test Plan

Unit Tests

go test ./internal/agentbuilder
go test ./...
go vet ./...

E2E Tests (Docker required)

cd e2e && ./docker-test.sh

• Unit tests pass (go test ./...)
• E2E tests pass (cd e2e && ./docker-test.sh)
• Manually tested locally

E2E was not run because this is a focused prompt-composition change with package and full Go test coverage.

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🤖 Automated Checks

The following checks run automatically on this PR:

Check	Status	Description
Check PR Cognitive Load	⏳	PR should stay within 400 changed lines (additions + deletions) or use size:exception
Check Issue Reference	⏳	PR body must contain Closes/Fixes/Resolves #N
Check Issue Has status:approved	⏳	Linked issue must have been approved before work began
Check PR Has type:* Label	⏳	Exactly one type:* label must be applied
Unit Tests	⏳	go test ./... must pass
E2E Tests	⏳	cd e2e && ./docker-test.sh must pass

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✅ Contributor Checklist

• PR is linked to an issue with status:approved
• PR stays within 400 changed lines, or I have requested/obtained maintainer-applied size:exception with rationale documented
• I have added the appropriate type:* label to this PR
• Unit tests pass (go test ./...)
• E2E tests pass (cd e2e && ./docker-test.sh)
• I have updated documentation if necessary
• My commits follow Conventional Commits format
• My commits do not include Co-Authored-By trailers

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💬 Notes for Reviewers

The important boundary is intentionally narrow: XML-like tags are for volatile prompt payloads only. SKILL.md remains Markdown because it is the durable human-authored and agent-consumed source of truth.

This hardens delimiter breakout, not prompt injection generally. User-controlled text can still contain persuasive instructions inside <user_request>, but it can no longer close that wrapper and appear as a separate authoritative section.

[Copilot]

Pull request overview

This PR hardens the internal/agentbuilder skill-generation prompt composition by introducing explicit XML-like wrappers around volatile/dynamic prompt payloads and escaping dynamic values to prevent wrapper delimiter breakout, while preserving the contract that the model must output raw SKILL.md Markdown.

Changes:

• Wrap installed-agent context, SDD context, and the user request in <installed_agents>, <sdd_context>, and <user_request> tags.
• Escape dynamic values via html.EscapeString to prevent </...> delimiter breakout from user/config-provided content.
• Update and extend unit tests to assert wrapper presence/absence and verify breakout-hardening behavior.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
internal/agentbuilder/prompt.go	Adds XML-like wrappers around volatile prompt sections and escapes dynamic values.
internal/agentbuilder/prompt_test.go	Updates prompt composition assertions and adds delimiter-breakout regression tests.

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