feat: implement tweaks plan - modals, smart shell, spinner, /models, debug log, skills

Phase 1: Permission modal dialog, session resume modal, HistoryInput with
up/down arrow cycling, remove "You:" echo from chat log, LLM client cleanup
on unmount.

Phase 2: Smart shell auto-approve using allowed/denied command lists from
ToolsConfig, animated thinking spinner with live token count in status bar.

Phase 3: /models slash command (list + switch), CLI directory positional
argument, JSONL debug logger with rotation.

Phase 4: Skills system with SkillsManager, load_skill LLM tool, /skills
listing, skill invocation via slash commands, system prompt integration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/services/skills.py

@@ -0,0 +1,71 @@
+"""Skills manager — scans for and loads skill markdown files."""
+
+from __future__ import annotations
+
+import logging
+from pathlib import Path
+
+from pydantic import BaseModel
+
+from app.models.config import SkillsConfig
+
+logger = logging.getLogger(__name__)
+
+
+class Skill(BaseModel):
+    """Metadata for a discovered skill file."""
+
+    name: str
+    description: str
+    path: Path
+
+
+class SkillsManager:
+    """Discovers, indexes, and loads skill files from configured directories."""
+
+    def __init__(self, config: SkillsConfig, workspace_root: Path) -> None:
+        self._config = config
+        self._workspace = workspace_root
+        self._skills: dict[str, Skill] = {}
+        self._scan()
+
+    def _scan(self) -> None:
+        """Scan configured directories for .md skill files."""
+        for skill_dir in self._config.directories:
+            resolved = (self._workspace / skill_dir) if not skill_dir.is_absolute() else skill_dir
+            if not resolved.is_dir():
+                logger.debug("Skills directory does not exist: %s", resolved)
+                continue
+            for md in sorted(resolved.glob("*.md")):
+                name = md.stem
+                desc = self._extract_description(md)
+                self._skills[name] = Skill(name=name, description=desc, path=md)
+                logger.debug("Discovered skill: %s (%s)", name, desc)
+
+    @staticmethod
+    def _extract_description(path: Path) -> str:
+        """Extract the first non-blank, non-heading line as the description."""
+        for line in path.read_text().splitlines():
+            stripped = line.strip()
+            if stripped and not stripped.startswith("#"):
+                return stripped
+        return "(no description)"
+
+    def list_skills(self) -> list[Skill]:
+        """Return all discovered skills."""
+        return list(self._skills.values())
+
+    def load_skill(self, name: str) -> str | None:
+        """Load the full content of a skill by name. Returns None if not found."""
+        skill = self._skills.get(name)
+        return skill.path.read_text() if skill else None
+
+    def get_system_prompt_snippet(self) -> str:
+        """Generate a snippet for the system prompt listing available skills."""
+        if not self._skills:
+            return ""
+        lines = ["\nAvailable skills (invoke with /skill-name):"]
+        for s in self._skills.values():
+            lines.append(f"  - /{s.name}: {s.description}")
+        lines.append("To use a skill's full instructions, call the load_skill tool.")
+        return "\n".join(lines)