Bump agents chart to v0.7.5

[casey-brooks]

Summary

• Bumps agents_chart_version from 0.7.4 to 0.7.5.
• 0.7.5 is the minimal agents chart/image release that contains Fix agent organization membership authorization agents#56 (1be4b3f), which writes and cleans up the identity:<agent_id> member organization:<org_id> OpenFGA tuple.
• No docs changes needed; this only updates the pinned chart/image version.

Closes #510

Test & Lint Summary

• terraform fmt -check -recursive: passed with no formatting changes required
• terraform -chdir=stacks/platform init -backend=false -input=false: passed
• terraform -chdir=stacks/platform validate: passed
• ./apply.sh -y: blocked by local rootless Docker runtime before deployment; k3d cluster creation failed because privileged sysfs mounts are not permitted in this environment (operation not permitted mounting sysfs)

Tests: 0 passed / 0 failed / 0 skipped (no test suite exists for this Terraform-only change).
Linting passed with no errors.

[casey-brooks]

Test & Lint Summary

• terraform fmt -check -recursive: passed with no formatting changes required
• terraform -chdir=stacks/platform init -backend=false -input=false: passed
• terraform -chdir=stacks/platform validate: passed
• ./apply.sh -y: blocked by local rootless Docker runtime before deployment; k3d cluster creation failed because privileged sysfs mounts are not permitted in this environment (operation not permitted mounting sysfs)

Tests: 0 passed / 0 failed / 0 skipped (no test suite exists for this Terraform-only change).
Linting passed with no errors.

[noa-lucent]

Review complete: the platform stack now pins the agents chart/image default to 0.7.5, and I verified the v0.7.5 agents tag points at commit 1be4b3f containing the agent org-membership tuple fix. Local checks passed: git diff --check, terraform fmt -check -recursive, terraform init -backend=false, and terraform validate. Approving.