docs(readme): tone down bold-headline paragraphs and marketing language

[killertcell428]

Summary

The README had drifted into a pattern of paragraph-opening bold "headlines" (**Two-layer defense —**, **On the audit gap.**, **MCP tool poisoning** —, ...) plus some self-referential marketing phrasing ("Aigis is the layer that answers it"). For a security-team audience this reads as more sales pitch than technical documentation.

• Replaced bold paragraph-openers with real ### subheadings where they were acting as section breaks, or plain sentences where they weren't
• Cut bold runs inside single sentences down to one emphasis point where one is still useful (several sentences had 3-4 bolded phrases)
• Rewrote the v2.0 section to lead with what was actually built (aigis profile build reads one role file, writes two permission files) instead of the "stops competing on pattern count" framing, which didn't explain the feature
• Dropped the "260+ patterns" count, which contradicted the v2.0 framing that Aigis isn't chasing a bigger rule count
• Same edits applied in parallel to README.md and README.ja.md

Not changed

• No structural changes: existing anchor IDs (#for-security-teams, #why-aigis, #limits, #learn-more, #quick-start) are untouched — new ### subheadings were added underneath them, not replacing them
• No content/claims changed, only how they're phrased

Test plan

• Anchor IDs referenced by the top nav links still exist (checked via grep)
• Docs-only change, no code touched