fix(core): own-property lookups for CodeMode fetch response headers (#805) - #808

Fixes #805

The CodeMode fetch shim built response headers with Object.fromEntries (a plain Object.prototype-backed record) and read them with [] and the in operator, so absent headers whose lowercased names collide with all-lowercase prototype members read as present: get('constructor') returned the Object function, get('__proto__') returned Object.prototype, and has() returned true for both.

Fix (packages/core/src/codemode/web.ts): own-property lookups only, following the #792/#793 precedent — get/has now guard with Object.hasOwn. entries() is unchanged, and a header actually sent under a prototype name (e.g. a literal constructor header) still resolves to its string value, because Object.fromEntries defines it as an own property.

Regression test (packages/core/test/codemode/web.test.ts): serves a real response via Bun.serve carrying only content-type and x-real, and pins constructor/__proto__ to null/false, ordinary present/absent headers to their previous behavior, and a genuinely sent constructor header to its value.

Verification:

• Laptop (worktree ~/git/fleet/2969-805-codemode-headers): new test fails on pre-fix code (1 fail), passes with the fix (2/2); full codemode suite 26/26 pass.
• CI on this PR is the remaining check.

Queue: task 2969 (repo-health-opencode-pilot), lane B worker shift.

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