refactor: reduce complexity of main() dispatch in src/main.rs

[github-actions]

What was complex

main() in src/main.rs was flagged by clippy::too_many_lines at 496/100 lines. It contained a large top-level match over Commands with several arms embedding substantial nested logic inline:

• Commands::Execute { .. } — an if/else deciding between --prepare-custom-agent-output validation/materialization and normal Stage 3 safe-output execution.
• Commands::Secrets { action } — a nested match action { Set | List | Delete } each building an options struct and dispatching.
• Commands::Graph { subcommand } — a nested match subcommand { Dump | Deps | Outputs }.

What changed

Extracted each of these into standalone, purpose-named async helper functions so main()'s match becomes a flat, one-line-per-arm dispatch table:

• dispatch_execute(ExecuteArgs) -> Result<()> — new ExecuteArgs struct carries the raw CLI fields; the function contains the --prepare-custom-agent-output validation/branching that was previously inline.
• dispatch_secrets(SecretsCmd) -> Result<()> — the Set | List | Delete match, unchanged in behavior.
• dispatch_graph_command(GraphCmd) -> Result<()> — the Dump | Deps | Outputs match, unchanged in behavior.

No public CLI behavior, flags, or error messages changed — this is purely a structural extraction to improve readability and testability.

Before / after

• Before: main() — 496/100 lines (clippy too_many_lines).
• After: main() — 351/100 lines (still above the 100-line lint threshold; further reduction of the remaining large match arms is left for a future pass).

Verification

• cargo build succeeds.
• cargo test — full suite passes, no failures.
• cargo clippy --all-targets --all-features — clean (one pre-existing, unrelated warning in create_work_item.rs).

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