story-kit: create 328_refactor_split_commands_rs_into_individual_command_handler_modules

.story_kit/work/1_backlog/328_refactor_split_commands_rs_into_individual_command_handler_modules.md

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+---
+name: "Split commands.rs into individual command handler modules"
+---
+
+# Refactor 328: Split commands.rs into individual command handler modules
+
+## Current State
+
+- TBD
+
+## Desired State
+
+commands.rs is 1,947 lines with 9 command handlers (help, status, ambient, git, htop, cost, show, overview, delete) plus all their tests in one file. Split into:
+- commands/mod.rs — command registry, dispatch, strip_bot_mention, BotCommand/CommandContext/CommandDispatch structs
+- commands/status.rs — handle_status, build_pipeline_status, read_stage_items, story_short_label
+- commands/cost.rs — handle_cost, extract_agent_type
+- commands/git.rs — handle_git
+- commands/ambient.rs — handle_ambient
+- commands/show.rs — handle_show
+- commands/overview.rs — handle_overview, find_story_merge_commit, get_commit_stat, extract_diff_symbols, parse_symbol_definition
+- commands/help.rs — handle_help
+- Tests split into corresponding test modules
+
+## Acceptance Criteria
+
+- [ ] commands.rs split into focused handler modules under matrix/commands/
+- [ ] Registry and dispatch remain in mod.rs
+- [ ] Each handler module contains the handler function and its tests
+- [ ] All existing tests pass without modification to test logic
+- [ ] No public API changes — try_handle_command still works the same way
+
+## Out of Scope
+
+- TBD