Reconcile Scala-version references: README/AGENTS say 3.9, project.scala/DESIGN say 3.10

[sideeffffect]

Severity: low · Category: docs

Evidence

• README.md:18 and AGENTS.md:23 say 3.9.0-RC1-…-NIGHTLY (AGENTS also references "Scala 3.9 CC" at :103-130).
• project.scala:1 and docs/DESIGN.md:7 pin 3.10.0-RC1-bin-20260607-dec42ae-NIGHTLY.

Problem
Three of the four version mentions are a major-version behind the actual pin; a user copying the README requirement targets the wrong nightly line.

Suggested fix
Point README/AGENTS at project.scala as the single source of truth (e.g. "Scala 3.10.0-RC1 nightly — see project.scala") rather than hardcoding 3.9.0; update the "Scala 3.9 CC" prose.

[sideeffffect]

Confirmed the split: project.scala:1 and docs/DESIGN.md:7 both pin 3.10.0-RC1-bin-20260607-dec42ae-NIGHTLY, while README.md:18 and AGENTS.md:23 still say 3.9.0-RC1-…-NIGHTLY, and AGENTS.md:104/:127 reference "Scala 3.9 CC" / "in Scala 3.9". Three of four mentions are a major version behind the actual pin.

Fix by making project.scala the single source of truth and not hardcoding a line in prose:

• README.md:18 — change
  - Scala \3.9.0-RC1-…-NIGHTLY` (capture checking / safe mode; see `project.scala`)to- Scala 3 nightly (capture checking / safe mode) — exact version pinned in `project.scala`(drop the literal3.9.0` so it can't drift again).

• AGENTS.md:23 — change the parenthetical (\3.9.0-RC1-bin-*-NIGHTLY` or later)to(see `project.scala` for the pinned nightly)`.

• AGENTS.md:104,127 — the "Scala 3.9 CC" / "in Scala 3.9" prose describes capture-checking behaviour that isn't version-specific; reword to "Scala 3 nightly CC" / "in nightly CC" so it survives future bumps.

This also dovetails with #22 (a Compatibility section): once the exact nightly lives in project.scala and is surfaced there, every other doc can link rather than copy it.