docs: v2.0.0 release readiness (HISTORY + #687 workaround)

[jepegit]

Summary

• Fold HISTORY.md into ## [2.0.0] - 2026-07-26 (support matrix, 12-month v1.x window, soak tags).
• Document OpenSSH Host alias workaround for remote scp:// / sftp:// paths (Regression: scp:// SSH-config Host aliases no longer resolve after UPath OtherPath switch #687 does not block ship).
• Refresh Stage-3 release guiding docs / branching checklist after rc1+rc2 soak.

Gate evidence (master tip)

• CI essential + full green: https://github.com/jepegit/cellpy/actions/runs/30204611332
• Benchmarks fail-band green (warn-band accepted, same class as rc1): https://github.com/jepegit/cellpy/actions/runs/30204611339
• Pin: cellpycore==0.2.4

After merge (D3 — not in this PR)

1. Tag v2.0.0 from clean master → watch release.yml → PyPI
2. Bump conda-forge cellpy-feedstock
3. Close Stage 3.17: cellpy 2.0.0 release checklist (benchmark acceptance, gates, tag) #574 / update Stage 3: cellpy 2.0.0 assembly (tracking issue) #575

Refs #574

Test plan

• uv run pytest -m essential on this branch
• CI green on the PR
• After merge: tag + publish + feedstock (manual)

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