Wire up completion keyboard navigation and add AGENTS.md

- Tab/Enter confirms the selected completion item
- Up/Down arrows navigate the completion menu
- Escape dismisses the menu (via existing dismiss_lsp_overlays)
- Editor intercepts keys via completion_intercept_keys flag when menu is showing
- Add AGENTS.md with LLM-powered predictive autocomplete idea

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

AGENTS.md

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+# Galaxy AI Agents - Ideas & Future Work
+
+## LLM-Powered Predictive Autocomplete
+
+**Idea:** As the user types in the code editor, stream the current context (surrounding code, file structure, recent edits) to an LLM and predict what they're about to write — offering inline ghost-text completions similar to GitHub Copilot.
+
+**Scope options:**
+- By line (predict the rest of the current line)
+- By function (predict the full function body)
+- By class/module (predict structural code)
+
+**Challenges:**
+- Latency: can't hit the LLM on every keystroke. Need aggressive debouncing (500ms+), speculative pre-fetching, and streaming partial results.
+- Cost: high token volume. May need a small/fast model (Haiku) for inline suggestions with a larger model for multi-line predictions.
+- Context window: need to efficiently pack relevant context (current file, imports, related types, recent edits) without blowing the token budget.
+- Cancellation: must cancel in-flight requests when the user keeps typing past the prediction point.
+- UX: ghost text rendering, Tab to accept, partial accept (word-by-word), dismiss on divergence.
+
+**Possible approaches:**
+- Debounce + streaming: wait 500ms after last keystroke, stream tokens as they arrive, render as ghost text
+- Predictive pre-fetch: on function signature completion or newline, proactively request the likely next block
+- Local model: run a small code model locally for instant line completions, use cloud model for multi-line
+- Hybrid: use LSP completions for symbol-level, LLM for line/block-level predictions
+
+**Integration points in Galaxy:**
+- `app/src/code/completion.rs` — extend the completion state machine with an LLM provider
+- `crates/ai/` — existing Bedrock/LLM infrastructure can be reused
+- Editor decoration system — for rendering ghost text (similar to inlay hints)