docs: point Git install references to master

[knqiufan]

Closes #1205

Rationale

The repository's main branch still contains legacy PowerMem (package metadata: powermem), while current PowerContext development is on master. The documented Git install ref therefore cannot install the requested powercontext package.

Changes

• Point all documented PowerContext Git-install and Codex-plugin refs from main to master.
• Update the package documentation URL to the master branch.
• Keep English and Chinese instructions aligned.

User-facing impact

Following the documented command now resolves PowerContext instead of the legacy PowerMem branch.

Validation

• In an isolated uv tool environment, the original @main command resolved 9d1b484 and failed with Package metadata name 'powermem' does not match given name 'powercontext'.
• The corrected @master command resolved c870f81, installed powercontext==0.0.1, and its CLI ran successfully. The isolated test installation and cache were removed afterward.
• git diff --check passed.
• uv run zensical build -s was attempted but timed out during first-run environment preparation after 244 seconds, without a build diagnostic.

[PsiACE]

Thank you for your support of powermem and powercontext.