feat: add --version and init subcommands (depends on #4)

[tupe12334]

Summary

Builds on #4 (feat/testable-main). Adds CLI subcommands, contributor docs, doc fixes, and schema fixes:

Binary subcommands (both documented in Installation.md but not implemented):

• steplock --version (also -V) — prints steplock 0.1.0
• steplock init — creates .steplock/checklists/ in the current directory; idempotent

Docs:

• Add CONTRIBUTING.md — setup, project layout, test/coverage/lint commands, hook setup, PR process
• Fix Architecture.md blank title (# → # Architecture)
• Fix Architecture.md config path (steplock.toml → .steplock/checklists/*/config.toml)
• Fix Installation.md cargo install command (steplock → steplock-core)
• Clarify packages/README.md — adds Status column showing Available vs Planned

Rustdoc:

• Fix 7 unresolved link to [*] warnings in flow.rs doc comments (escape [*] as backtick-quoted)
• Add rustdoc to ChecklistConfig, Reset, FlowGraph, parse_mmd, parse_config, load_state, is_complete

Schema:

• Fix session-state.schema.json — remove minItems: 1 from transitions (empty on checklist completion is valid)

Other:

• Add *.profraw to .gitignore (coverage run artifacts)

Coverage: 96.78% lines overall (remaining gaps are process::exit paths — inherently untestable in unit tests)

Merge order: merge #5 and #4 first, then this PR. Or merge this PR directly — it includes all commits from #4.

Test plan

• cargo test --all-targets passes (11 tests in bin, 80 total)
• RUSTDOCFLAGS="-D warnings" cargo doc produces no warnings
• steplock --version prints steplock 0.1.0
• steplock init creates .steplock/checklists/ and is idempotent
• CI passes on this PR