docs: fix Poetry activation and uv environment setup (bounty claim)

[benthepythondev00]

The installation guides omit the activated environment required by uv pip install, and the Poetry tab uses poetry shell, which is no longer included in a default Poetry 2.x installation.

Link the existing virtual-environment instructions before pip/uv installation in the main guide, development setup and FAQ. Replace poetry shell with eval "$(poetry env activate)" for Bash/Zsh, explain evaluation, and provide the documented PowerShell equivalent.

Validation:

• Reproduced No virtual environment found with uv 0.11.21 in an isolated dry run without an activated/discoverable environment. With the workspace Python 3.11 environment activated, uv pip install --dry-run --no-config swarms resolved 81 packages successfully. This was dependency resolution only; no full framework install or model call.
• Reproduced the old Poetry command's exit 1 with Poetry 2.4.3. Ran its replacement in Bash and Zsh against an isolated Python 3.11 environment; both asserted the expected interpreter prefix.
• Checked PowerShell syntax against Poetry's official guide; PowerShell was not executed.
• mint broken-links: no broken links. git diff --check: passed. Visually inspected the affected local preview sections and verified the environment anchor exists.

Bounty claim: please consider Bronze ($1–$5) under the documentation program, for these minor installation corrections. Tier and payment are subject to your review after merge. PayPal preferred if awarded; I can provide payment details privately.