fix(dag): resolve global dag-prompts from the configured config directory

[LeXwDeX]

Problem

Global DAG prompt lookup hardcodes ~/.config/opencode/dag-prompts, ignoring both OPENCODE_CONFIG_DIR and XDG_CONFIG_HOME. Other global DAG and memory assets consistently resolve through Flag.OPENCODE_CONFIG_DIR ?? Global.Path.config.

User Impact

Users with redirected config directories can successfully install/sync global prompt assets into the configured opencode directory, but prompt_template: { id } cannot resolve them and reports the misleading validation error prompt id ... does not resolve. This breaks every non-default XDG/config-dir setup.

Evidence

• packages/opencode/src/dag/templates/resolve.ts:95: hardcoded path.join(os.homedir(), ".config", "opencode", "dag-prompts", ...).
• packages/opencode/src/dag/workflows.ts:152, packages/opencode/src/dag/config.ts:116, and packages/opencode/src/memory/config.ts:167: sibling global assets use Flag.OPENCODE_CONFIG_DIR ?? Global.Path.config.
• packages/core/src/plugin/command/dag-template-update.txt:16-24: the command contract explicitly requires OPENCODE_CONFIG_DIR and XDG support.
• packages/opencode/src/dag/validation.ts:999-1000: unresolved prompt diagnostic.
• packages/opencode/src/dag/templates/resolve.ts:42-44,97-102: safe-ID validation and project-first precedence that must remain unchanged.

Fix Requirements

1. Resolve the global path with path.join(Flag.OPENCODE_CONFIG_DIR ?? Global.Path.config, "dag-prompts", ${id}.md).
2. Preserve isSafeTemplateId, project-scoped precedence, interpolation, and diagnostics.
3. Update the nearby comment describing the global directory.
4. Add a focused OPENCODE_CONFIG_DIR redirection test in packages/opencode/test/dag/dag-templates.test.ts.

Non-goals

• Do not add migration or symlink behavior.
• Do not change project-scoped lookup, sanitization, interpolation, or diagnostic wording.

Acceptance Criteria

• With OPENCODE_CONFIG_DIR=/tmp/x and /tmp/x/dag-prompts/foo.md present, prompt id foo resolves that file.
• Default behavior remains unchanged when no override is set, including XDG resolution through Global.Path.config.
• os.homedir is no longer used by dag/templates/resolve.ts.

Likely Code Locations

• packages/opencode/src/dag/templates/resolve.ts
• packages/opencode/test/dag/dag-templates.test.ts

Verification

cd packages/opencode && bun test test/dag/dag-templates.test.ts && bun typecheck
git grep -n "os.homedir" -- packages/opencode/src/dag/templates/resolve.ts

[LeXwDeX]

Delivered via #382 (merged to dev); promoting to main via #383.