feat: implement typed lifecycle command format support

[pofallon]

Summary

Implements full support for three lifecycle command formats per the DevContainer specification:

• String format: Shell-interpreted commands (/bin/sh -c in container, sh -c on host)
• Array format: Exec-style direct OS invocation without shell wrapping
• Object format: Named parallel commands executing concurrently with full error aggregation

What's Changed

Core Type System

• Introduce LifecycleCommandValue enum with Shell(String), Exec(Vec<String>), Parallel(IndexMap<String, LifecycleCommandValue>) variants
• Implement spec-compliant parsing via from_json_value() with type validation
• Thread typed commands through aggregation and execution pipeline for all 6 lifecycle phases

Execution

• Format-aware execution in both container and host paths
• Parallel execution via tokio::JoinSet — all entries run concurrently, all complete before phase result reported (no early cancellation per spec)
• Variable substitution applied element-wise for arrays, recursively for objects
• Capture stdout/stderr from parallel commands for diagnostics
• Output attributed via [key] prefixes in progress events per entry

Code Quality Fixes

• Replace 4 unreachable!() calls in runtime paths with proper DeaconError::Lifecycle returns
• Return validation errors for invalid array entries inside objects (matches top-level behavior)
• Filter empty strings/arrays in object entries as no-ops at parse time
• Document serde_json preserve_order dependency for object key ordering
• Extract duplicated workspace folder derivation to shared::derive_container_workspace_folder()
• Log warn! on poisoned progress tracker mutex instead of silent swallow
• Use sentinel values in host-only dummy config instead of empty strings
• Remove initializeCommand from run-user-commands (host-only, belongs to up only)
• Document feature lifecycle aggregation gap in run-user-commands with TODO

Tests

• 4 new concurrency tests proving parallel execution is truly concurrent (~500ms for two 500ms tasks)
• Error aggregation tests verifying all parallel entries complete despite individual failures
• Comprehensive format × phase integration tests
• 1758 tests passing, 0 failures

Known Limitations

• run-user-commands feature gap: Feature-defined lifecycle commands are not yet aggregated (documented with TODO). Requires resolving features from container image metadata labels.
• Parallel output terminal prefixing: Output captured in CommandResult.stdout/stderr but not line-prefixed to terminal in real-time (inherited stdio model constraint).

Spec Compliance

✓ All 3 formats work for all 6 lifecycle phases
✓ Exec-style arrays bypass shell — arguments passed literally
✓ Parallel objects run concurrently, wait for all, aggregate failures
✓ Variable substitution preserves structure (element-wise / recursive)
✓ Empty/null commands filtered as no-ops at every level
✓ Per-entry progress events with {phase}-{key} command IDs

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