docs: say how to replace an older install before specify init

[kartsan03]

Description

In #4994 a fresh-looking install of specify-cli reports CLI 0.1.6, and specify init fails with No such option: --integration, then No matching release asset found for claude. The README tells you to run uv tool install specify-cli, and when specify-cli is already installed, that command keeps the installed version. uv still exits 0 and prints Installed 1 executable: specify, so it looks like it worked. uv tool upgrade specify-cli doesn't help either when the old install came from a Git tag. PyPI's oldest specify-cli release is 0.9.4, so 0.1.6 must be an earlier Git install that was left in place.

Reproduced with uv 0.12.21, using a scratch UV_TOOL_DIR/UV_TOOL_BIN_DIR:

$ uv tool install specify-cli --from git+https://github.com/github/spec-kit.git@v0.1.6
$ specify version               # CLI Version 0.1.6
$ uv tool install specify-cli   # exit 0: "Checked 22 packages ... Installed 1 executable: specify"
$ specify version               # CLI Version 0.1.6
$ uv tool upgrade specify-cli   # "Nothing to upgrade"
$ specify init demo --integration claude
No such option: --integration
$ specify init demo --ai claude --script sh --ignore-agent-tools
No matching release asset found for claude (expected pattern: spec-kit-template-claude-sh)
Initialization failed: 1
$ uv tool install --force specify-cli
$ specify version               # CLI Version 1.2.1

The same happens over a PyPI install: with 1.1.3 installed, uv tool install specify-cli keeps 1.1.3. pipx install specify-cli keeps it too (it does say to pass --force), and pip install specify-cli stops at Requirement already satisfied.

This PR adds that to the two places that show the install commands:

• README.md, under the Get started commands: if Spec Kit is already installed, check specify version and reinstall with uv tool install --force specify-cli before specify init.
• docs/installation.md, under "Install from PyPI": the same, with the pipx and pip equivalents (the commands docs/install/pypi.md already gives for upgrades), plus the two error messages an old CLI prints, so the page can be found by searching for them.

Docs only, +6 lines. Refs #4994

Testing

• Tested locally with uv run specify --help
• Ran existing tests with uv sync && uv run pytest
• Tested with a sample project (if applicable)

I didn't run the test suite, since nothing outside the two Markdown files changed. I ran every command in the description, plus the 1.1.3, pipx and pip cases, against PyPI and GitHub with uv 0.12.21, each in its own scratch tool directory.

AI Disclosure

• I did not use AI assistance for this contribution
• I did use AI assistance (fill in the disclosure below)

AI disclosure: Claude Code (model: claude-opus-5-5, default reasoning effort, autonomous) reproduced the install behaviour with uv, pipx and pip, wrote the docs change and this description, and made the commit (it carries an Assisted-by trailer). Extent: docs only.