Add `viz` CLI command (ASCII + DOT export)

[dgenio]

Context

Split from #33 (CLI meta-issue). This issue adds flow visualization export to the CLI established in #44.

What to do

1. ASCII visualization

chainweaver viz <flow_name> --format ascii produces terminal-friendly output:

Linear flows:

[double] → [add_ten] → [format_result]

DAG flows (when #10 is implemented):

          ┌─[branch_a]─┐
[start]──┤              ├──[merge]
          └─[branch_b]─┘

2. DOT (Graphviz) export

chainweaver viz <flow_name> --format dot produces valid DOT output:

digraph double_add_format {
    double -> add_ten;
    add_ten -> format_result;
}

This enables chainweaver viz flow --format dot | dot -Tpng -o flow.png without ChainWeaver depending on Graphviz.

3. Default format

--format ascii is the default.

Files to modify

• chainweaver/cli.py — add viz subcommand
• tests/test_cli.py — add visualization tests

Acceptance Criteria

• chainweaver viz <flow> --format ascii produces ASCII visualization for linear flows
• chainweaver viz <flow> --format dot produces valid DOT graph output
• ASCII output uses arrow notation (→) for step connections
• DOT output is syntactically valid (can be parsed by Graphviz)
• Exit codes: 0 = success, 1 = flow not found
• At least 3 test cases: ASCII output correctness, DOT output validity, missing flow error

Out of Scope

• Web-based visualization
• Interactive TUI
• DAG visualization (can be added when Design and implement DAG-based flow model with topological execution #10 lands — for now, linear flows only)
• SVG/PNG rendering (users pipe DOT output to Graphviz themselves)

Dependencies

• Implement CLI entry point with inspect command #44 (CLI entry point must exist first)
• Optional: Design and implement DAG-based flow model with topological execution #10 (DAG model) for non-linear visualization

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Part of CLI split from #33. See also: #44 (CLI entry + inspect), #45 (validate + check).