refactor: Config.Sandbox struct + Context pipeline for stateful sessions

[conroywhitney]

Summary

Complete refactor establishing proper separation between static config and dynamic runtime state, enabling stateful shell sessions where cd persists across commands.

Architecture

agents.yaml → Config.discover() → Context.from_config() → execute(cmd, ctx)
                                         ↓
                          %Context{current_path, sandbox_config, stdin}
                                         ↓
                    Expander → Executor → Commands → {output, ctx}

Key Changes

Config.Sandbox (static, immutable)

• allowed_paths - boundaries for validation (supports multiple roots + globs)
• home_path - where cd ~ goes, tilde expansion target

Commands.Context (dynamic runtime state)

• current_path - changes with cd, used for relative path resolution
• sandbox_config - reference to immutable Config.Sandbox
• stdin - cleared between commands to prevent leakage

TrumanShell.execute/2

• Accepts optional context parameter
• Returns {:ok, output, ctx} for state management
• Enables chaining: cd lib → ls shows lib contents

CLI Simplified

• Config only from agents.yaml (no env vars)
• validate-path <path> uses config directly via Context
• Clean usage: execute <cmd>, validate-path <path>, version

Breaking Changes

• TrumanShell.execute/1 now returns 3-tuple {:ok, output, ctx}
• YAML uses allowed_paths/home_path (not roots/default_cwd)
• CLI env vars removed (TRUMAN_CMD, TRUMAN_VALIDATE_PATH, TRUMAN_CURRENT_DIR)

Test Plan

• 549 tests + 91 doctests passing
• Credo clean (1 false positive on function ordering)
• Dialyzer clean
• LLM Review: 6 rounds, final 4/4 LGTM

LLM Review Summary

Round	Result	Key Changes
01	1/4	Initial struct refactor
02	2/4	Fix Expander glob/tilde semantics
03	2/4	Context flow cleanup
04	2/4	Config.Sandbox.from_yaml, 3-tuple returns
05	3/4	CLI uses config.sandbox, docs fixed
06	4/4	CLI simplified, stdin clearing, integration tests

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