feat: add --context flag for temporary config profile switching

[JannikSt]

Summary

• Adds --context/-c global flag to temporarily use a different config profile for a single command
• Similar to kubectl --context - allows switching between users/environments without running prime config use

Usage

# Instead of:
prime config use user1
prime sandbox create ...
prime config use user2
prime sandbox create ...

# You can now do:
prime --context user1 sandbox create ...
prime --context user2 sandbox create ...

# Or with short flag:
prime -c production pods list

Implementation

• Added --context option to main app callback
• Sets PRIME_CONTEXT env var which Config checks on init
• Added persist=False option to load_environment() for in-memory-only loading

Test plan

• prime --context production config view shows production config
• prime config view after still shows original config (not persisted)
• prime --context nonexistent ... shows helpful error with available contexts
• All existing tests pass

Closes #91

---

Note

Adds a global --context/-c flag that temporarily switches the config environment for a single command via PRIME_CONTEXT, backed by Config.load_environment(..., persist=False).

• CLI:
  • Add global --context/-c option in prime_cli/main.py to use a specific config environment per command.
  • Validates context (except production) against Config.list_environments() and prints available contexts on error.
  • Sets PRIME_CONTEXT so subcommands inherit the selected environment.
• Config:
  • Check PRIME_CONTEXT on init in core/config.py and load_environment(context, persist=False) for in-memory override.
  • Extend load_environment(name, persist=True|False) to support non-persistent loading for production and saved environments, including URL normalization and selective field updates.

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[pokgak]

this one a bit confusing because --context need to be explicitly before any subcommands like prime --context local sandbox list and not prime sandbox list --context=local. works but ideally we should be able to put it anywhere after prime.

not a big issue tho so approved.

[pokgak]

move this to top file?