docs: remove stale hard-coded default workflow count

Problem

README.md says Archon ships 19 default workflows, but the current CLI reports 31 bundled defaults. The hard-coded count has drifted from the product and gives new users a false inventory.

Why this matters

The README is the first setup and discovery path for many users. A stale workflow count makes the documented product surface look incomplete and will drift again as bundled workflows change.

Why now

The mismatch is reproducible on the current dev branch with archon doctor and archon workflow list.

Desired outcome

The README describes bundled workflows without a manually maintained numeric count, while continuing to direct users to archon workflow list as the authoritative catalog.

Invariants

• Do not change workflow loading or the bundled workflow set.
• Keep the existing workflow discovery command and surrounding guidance.
• Do not introduce a generated artifact or dependency for this documentation-only correction.

Acceptance

• README.md no longer asserts the stale hard-coded count.
• The wording remains accurate when workflows are added or removed.
• Documentation formatting checks pass.