chore: strip per-file license headers; LICENSE is single source of truth

[ChrisonSimtian]

Why

Per-file license headers duplicate a 4-line block across 614 source files. MIT compliance only requires the notice in distributed copies of the software — LICENSE at the repo root and README.md both already preserve matkoch's attribution, and every NuGet package declares MIT via PackageLicenseExpression in Directory.Build.props. So the per-file blocks are pure overhead.

The concrete trigger: the header URL https://github.com/ChrisonSimtian/Fallout/blob/main/LICENSE would have rotted across hundreds of files the moment the repo transfers to the Fallout-build org (#11). Better to consolidate once than to mass-edit a stale URL later.

One source of truth, cleaner source files, things live where they belong instead of being duplicated 614 times. #housekeeping

What

• Removed the 4-line // Copyright … Maintainers of Fallout. // Originally based on NUKE … block from 614 first-party .cs files under src/, tests/, build/, and the repo root.
• Preserved third-party headers: files under src/Persistence/Fallout.Persistence.Solution/ (inlined Microsoft vs-solutionpersistence) keep their // Copyright (c) Microsoft Corporation. headers — MIT requires it.
• Preserved auto-gen headers: *.Generated.cs tool wrappers and .verified.cs snapshot fixtures keep their existing auto-gen comments.
• Updated convention docs: AGENTS.md (rule chore(deps): patch vulnerable transitives (CVE-2026-33116, Scriban) #4 replaced with explicit no-headers rule), docs/agents/conventions.md, docs/architecture.md, CONTRIBUTING.md.
• Added CHANGELOG.md entry under [Unreleased] — 11.0 → ### Misc.

Legal check

• LICENSE preserves Copyright 2017-2025 Maintainers of NUKE (matkoch's attribution) alongside Copyright 2026 Maintainers of Fallout.
• README.md lines 34-38 explicitly credit Matthias Koch and the NUKE origin.
• Every NuGet package carries <PackageLicenseExpression>MIT</PackageLicenseExpression> + <Copyright>Copyright … Maintainers of Fallout</Copyright> via Directory.Build.props.
• Vendored MIT code retains its own copyright headers (untouched in this PR).

Verification

• git grep "Maintainers of Fallout" -- '*.cs' → 0 hits (was 614).
• git grep "Copyright (c) Microsoft Corporation" -- '*.cs' → unchanged (Persistence files preserved).
• ./build.sh Compile passes (verified on a parallel worktree carrying the same strip).
• BOM and CRLF/LF line endings preserved per-file by the strip script.

Test plan

• No first-party .cs file still contains "Maintainers of Fallout"
• All Microsoft-copyright files under Fallout.Persistence.Solution/ unchanged
• .Generated.cs and .verified.cs files unchanged
• Compilation clean (0 errors)
• CI green on this PR (pending)

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