When unique_milestone_ids is enabled, the LLM cannot generate random
suffixes itself. Previously only the first milestone got a correct ID
(pre-generated in TS), while subsequent milestones in multi-milestone
projects got bare M002/M003 without suffixes.
Added a gsd_generate_milestone_id tool that the LLM calls to get each
milestone ID. The tool scans disk for existing milestones and respects
the unique_milestone_ids preference, making it impossible to produce
wrong-format IDs.
Updated discuss, discuss-headless, and queue prompts to instruct the
LLM to use the tool instead of inventing milestone IDs.
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