fix(playbook): collapse blank lines around issue context partials

[chihsuan]

Context

symphony workflow preview rendered a stack of blank lines around the issue-context block whenever optional sections (description, comments, links, retry context) were empty, making the prompt harder to scan.

TL;DR

Use Liquid whitespace control so empty issue-context sections collapse instead of leaving stray blank lines.

Summary

• Inline the issue description directly after Description: so there is no blank line between the label and the body.
• Trim the Recent comments and Linked issues {% if %} blocks with {%-/-%} so they emit nothing when empty.
• Render continuation_context to an empty string when no retry attempt is set, instead of leaking surrounding newlines.

Alternatives

• Leave the blanks as-is: rejected, the rendered prompt is what the agent and authors read, so readability matters.
• Strip blank lines in the renderer post-pass: rejected, it would also collapse intentional spacing elsewhere; fixing the templates is targeted and idiomatic.

Test Plan

• make all
• mix test test/symphony_elixir/workflow_preview_test.exs test/symphony_elixir/prompt_builder_test.exs test/symphony_elixir/quality_gate/prompt_test.exs (+ core prompt cases) — green
• symphony workflow preview before/after: blank line after Description: gone; trailing gap cut from 6 blank lines to 1