feat: expose project-map.json bootstrap as idempotent simplicio-mapper index command

[wesleysimplicio]

Open this issue inside simplicio-mapper. Read the repo (AGENTS.md,
README.md, existing CLI subcommands) before implementing. Adapt naming /
module layout to what's already there.

Why

simplicio-sprint (issue wesleysimplicio/simplicio-sprint#273) needs to invoke
simplicio-mapper index <repo> automatically when the target repo lacks
.simplicio/project-map.json. The mapper currently has indexing logic — this
issue is about making it a stable, scriptable command that sendsprint can call
on every run without paying full cost.

What SendSprint expects

• A single entry point — simplicio-mapper index <path> (or the equivalent
  subcommand your CLI already exposes).
• Idempotent: re-running with a fresh index detects via repo hash /
  mtime / git rev and short-circuits in <200 ms.
• Quiet by default: only emit progress when --verbose; stay silent on
  successful no-op so it doesn't pollute sendsprint logs.
• Structured output: --json produces a stable schema sendsprint can
  parse (paths produced, item counts, skipped reason).
• Clear exit codes: 0 = wrote/refreshed, 1 = failure, 2 = skipped
  (already fresh).

Acceptance

• simplicio-mapper index . produces or refreshes
  .simplicio/project-map.json (and precedent-index.json if you ship it).
• Re-run on an unchanged repo: exit code 2, <200 ms.
• --json is a stable contract documented in README.md.
• Tests cover the three paths (fresh / refresh / no-op).

Out of scope

• Changing the JSON schema of project-map.json (sendsprint already consumes it).
• The bootstrap from sendsprint side — that's feat: auto-bootstrap simplicio-mapper index quando .simplicio/ ausente simplicio-sprint#273.

Refs: wesleysimplicio/simplicio-sprint#273, wesleysimplicio/simplicio-sprint#262, wesleysimplicio/simplicio-sprint#279.

[wesleysimplicio]

Implemented and published on main.

Evidence:

• PR feat: add idempotent mapper index command #89 added the idempotent simplicio-mapper index <path> command and contract tests.
• Follow-up commit 3ca706b fixed the JSON path contract on Windows and preserves counts in .simplicio/index-state.json so skipped responses still expose item counts.
• Validation: python -m unittest discover -s tests\python -p "test_*.py" -> 15 tests OK, 1 skipped; git diff --check -> clean.

Closing as completed.