docs: say anchors are ready for external commitment, not committed

[yesbert]

What this changes

Three sentences in README.md and docs/index.md said the event chain is "anchored outside your database". They now say the chain carries periodic anchors that you can commit outside your database.

Why

The tamper-evident-streams specification states that external anchoring is a seam, not a shipped integration: the framework records anchors and carries a field for an external commitment reference, and the consumer chooses the external target and submits to it. The concept page already says this; the README and the site landing page overclaimed. A reader who compares the two finds a promise the framework does not make.

How it was verified

Docs only, no behaviour change, no spec delta. The governing guarantee is unchanged in openspec/specs/tamper-evident-streams/spec.md (External anchoring is a seam, not a shipped integration).

• ./scripts/local-gauntlet.sh passes

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