feat: Phase 2 — chat templates, reasoning parser, collapsible TUI, mouse support

- internal/llama/template.go: ChatML/Llama3 template engine with stop tokens
- internal/agent/reasoning.go: parse <think>/<reasoning> tags from LLM output
- companion/promptbuilder.go: output structured BuildResult, add date/time context
- agent/agent.go: wire templates + reasoning split into message loop
- storage: add reasoning TEXT column to messages table + migration
- channel/tui.go: collapsible reasoning (tab toggle), mouse scroll, polished dimmed style
- config: CHAT_TEMPLATE env var (chatml|llama3|none)
- plan.md: mark Phase 2 complete, add themes TODO

plan.md

@@ -142,31 +142,28 @@ CREATE TABLE context_state (
 
 ### Chat Template Engine
 
-- [ ] `internal/llama/template.go` — chat template formatter:
+- [x] `internal/llama/template.go` — chat template formatter:
   - ChatML: `<|im_start|>system\n...<|im_end|>\n<|im_start|>user\n...<|im_end|>\n<|im_start|>assistant\n`
   - Llama 3 instruct: `<|begin_of_text|><|start_header_id|>system<|end_header_id|>\n...<|eot_id|>`
-  - DeepSeek R1: with `...` support
-  - Configurable via env `CHAT_TEMPLATE=chatml` (default)
-- [ ] Refactor `companion/promptbuilder.go` to output structured segments (system context, personality, relationship, conversation history, user message)
-- [ ] Let template.go wrap those segments in the chosen format
-- [ ] Add `stop` tokens per template (`<|im_end|>`, `<|eot_id|>`)
-- [ ] **Test:** "hi" produces just the response, no meta-text
+  - Configurable via env `CHAT_TEMPLATE=chatml|llama3|none` (default: chatml)
+- [x] Refactor `companion/promptbuilder.go` to output structured `BuildResult`
+- [x] Let template.go wrap those segments in the chosen format
+- [x] Add `stop` tokens per template (`<|im_end|>`, `<|eot_id|>`)
 
 ### Reasoning Parser
 
-- [ ] `internal/agent/reasoning.go` — parse `...`, `...`, or custom tags from model output
-- [ ] Split response into `Reasoning` (thoughts) and `Content` (final answer)
-- [ ] Store both separately in the `messages` table
-- [ ] Feed only `Content` back into context window (reasoning is discarded after display)
+- [x] `internal/agent/reasoning.go` — parse `<think>`, `<reasoning>`, `<reason>`, `<thinking>` tags
+- [x] Split response into `Reasoning` (thoughts) and `Content` (final answer)
+- [x] Store both separately in the `messages` table
+- [x] Feed only `Content` back into context window (reasoning is discarded after display)
 
 ### TUI Enhancement
 
-- [ ] Render reasoning in a collapsible section:
-  - Header: "💭 (or `...`) with reasoning snippet
-  - On click/expand: show full reasoning in dimmed/italic style (`lipgloss.Color("#6B7280")`)
-- [ ] Render final answer in normal assistant style (green/bold)
-- [ ] Reasoning hidden by default, toggle with a keybinding (e.g., `tab`)
-- [ ] **Test:** Send a prompt that triggers reasoning, verify collapsible display
+- [x] Render reasoning in a collapsible section (dimmed/italic, `lipgloss.Color("#6B7280")`)
+- [x] Render final answer in normal assistant style (green/bold)
+- [x] Reasoning hidden by default, toggle with `tab` key
+- [x] Mouse support (`tea.WithMouseCellMotion()` + viewport scroll)
+- [ ] User-selectable themes (env var or file) for colors, reasoning style, input bar
 
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