feat(system-prompt): add PREAMBLE.md workspace file as template variable for prompt profiles

[Cstewart-HC]

Summary

Add PREAMBLE.md workspace file support to the prompt profile system, enabling per-agent custom opening text via the existing {{template_variable}} mechanism introduced in #466.

Motivation

PR #466 introduces a comprehensive prompt profile system with template variables, section toggles, and model-based profile routing. A PREAMBLE.md workspace file (following the existing SOUL.md, BOOT.md, TOOLS.md pattern) would allow users to define custom preamble text as a file-based override, which is then exposed as a {{preamble}} template variable inside any prompt profile.

This was originally developed on feat/configurable-preamble but conflicts with #466's builder rewrite. Re-implementing as a template variable within the profile system is the correct architectural fit.

Requirements

moltis-config

• Add preamble_path() to crates/config/src/loader.rs — returns data_dir().join("PREAMBLE.md")
• Add load_preamble_md() and load_preamble_md_for_agent(agent_id) to crates/config/src/loader/workspace.rs — follows existing load_soul_md / load_boot_md fallback pattern:
  1. agents/{id}/PREAMBLE.md (agent-specific override)
  2. PREAMBLE.md (global fallback)
• Export from crates/config/src/lib.rs

moltis-agents

• Add preamble to PROMPT_TEMPLATE_VARIABLES array in crates/agents/src/prompt.rs
• Wire preamble text into PromptRenderContext and build_template_values() so {{preamble}} resolves at render time
• Add "preamble" to PromptSectionId enum if section-toggle support is desired

moltis-chat

• Add preamble_text: Option<String> to PromptPersona in crates/chat/src/types.rs
• Load preamble in crates/chat/src/prompt.rs via moltis_config::load_preamble_md_for_agent(agent_id)
• Thread preamble text through to the builder when calling build_system_prompt_with_profile

Tests

• Unit tests in moltis-agents for {{preamble}} template variable resolution
• Unit tests in moltis-config for file loading fallback chain (agent override → global)
• Integration test: preamble absent → default profile behavior unchanged

Documentation

• Add PREAMBLE.md to the workspace file documentation alongside SOUL.md, BOOT.md, etc.

Dependencies

• Blocked by: feat(system-prompt): add prompt profiles with CRUD, section options, and model overrides #466 (prompt profiles with CRUD, section options, and model overrides)
• Must rebase onto main after feat(system-prompt): add prompt profiles with CRUD, section options, and model overrides #466 merges

Design Notes

• The preamble file is user-editable (like all workspace markdown files)
• Fallback chain mirrors SOUL.md and BOOT.md: agent-level file takes priority over global
• When no PREAMBLE.md exists, the {{preamble}} variable resolves to empty string — no behavior change
• The existing load_preamble_md() / load_preamble_md_for_agent() implementations from the aborted feat/configurable-preamble branch can be reused directly (zero conflict with feat(system-prompt): add prompt profiles with CRUD, section options, and model overrides #466)

Original Branch

feat/configurable-preamble — archived after conflict analysis with #466. Conflict analysis saved at CONFLICT_ANALYSIS_PR466.md.