fix(engine): pin web-tree-sitter back to ^0.20.8, unblocking the engine build

[JSONbored]

Summary

The npm dependency bump in #9232 jumped web-tree-sitter from ^0.20.8 to ^0.26.11 — a breaking major-version range change. Parser.init() and the SyntaxNode export moved in the new major version, breaking packages/loopover-engine/src/miner/repo-map.ts's tsc build. This has been failing the engine build (and any CI job that builds it) for every PR based on main since that merge — confirmed reproducing on a fresh origin/main checkout.

This PR pins web-tree-sitter back to ^0.20.8 (the previously-working range) rather than migrating repo-map.ts to the new major-version API, to unblock CI immediately. A proper migration to 0.26.x (updating repo-map.ts for the new Parser.init()/SyntaxNode shape) can follow as a separate, deliberate PR.

Test plan

• npm run build --workspace @loopover/engine — clean (was failing before this pin)
• npx tsc --noEmit -p tsconfig.json --incremental false — clean across the whole repo

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

❌ 1 Tests Failed:

Tests completed	Failed	Passed	Skipped
22708	1	22707	21

View the top 1 failed test(s) by shortest run time

test/unit/repo-map.test.ts > test/unit/repo-map.test.ts

Stack Traces | 0s run time

Error: Cannot find package 'web-tree-sitter' imported from .../test/unit/repo-map.test.ts
 ❯ test/unit/repo-map.test.ts:3:1

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Serialized Error: { code: 'ERR_MODULE_NOT_FOUND' }

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